Function1: Added squarePulse

This function gives a value of one during a user-specified duration, and
zero at all other times. It is useful for defining the time range in
which an injection or ignition heat source or similar operates.

Example usage, scaling a value:

    <name>
    {
        type        scale;
        scale       squarePulse;
        start       0;
        duration    1;
        value       100;
    }

This function has been utilised in a number of tutorial fvOption
configurations to provide a specific window in which the fvOption is
applied. This was previously achieved by "timeStart" and "duration"
controls hard coded into the fvOptions themselves.

tutorials/heatTransfer/chtMultiRegionFoam/heatedDuct/constant/heater/fvOptions

@@ -21,9 +21,8 @@ options
     {
         type            semiImplicitSource;
 
-        timeStart       0;
-        duration        1e6;
         selectionMode   all;
+
         volumeMode      specific;
 
         sources

tutorials/heatTransfer/chtMultiRegionFoam/reverseBurner/constant/gas/fvOptions

@@ -15,6 +15,32 @@ FoamFile
 }
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
+ignition
+{
+    type            semiImplicitSource;
+
+    selectionMode   cellSet;
+    cellSet         ignition;
+
+    volumeMode      specific;
+
+    sources
+    {
+        h
+        {
+            explicit
+            {
+                type        scale;
+                scale       squarePulse;
+                start       0;
+                duration    1;
+                value       5e7; // kg/m/s^3
+            }
+            implicit    0;
+        }
+    }
+}
+
 radiation
 {
     type    radiation;

tutorials/incompressible/pimpleFoam/laminar/planarPoiseuille/constant/fvOptions

@@ -19,8 +19,6 @@ momentumSource
 {
     type            semiImplicitSource;
 
-    timeStart       0.0;
-    duration        1000;
     selectionMode   all;
 
     volumeMode      specific;

tutorials/lagrangian/reactingParticleFoam/filter/constant/fvOptions

@@ -48,8 +48,6 @@ massSource
 {
     type            semiImplicitSource;
 
-    timeStart       0.2;
-    duration        2.0;
     selectionMode   points;
     points
     (
@@ -58,29 +56,53 @@ massSource
 
     volumeMode      absolute;
 
+    squarePulse
+    {
+        type            scale;
+        scale           squarePulse;
+        start           0.2;
+        duration        2;
+    }
+
     sources
     {
         rho
         {
-            explicit    1e-4; // kg/s
+            explicit
+            {
+                $squarePulse;
+                value       1e-4; // kg/s
+            }
             implicit    0;
         }
 
         U
         {
-            explicit    (0 0.005 0);
+            explicit
+            {
+                $squarePulse;
+                value       (0 0.005 0);
+            }
             implicit    0;
         }
 
         h
         {
-            explicit    10;
+            explicit
+            {
+                $squarePulse;
+                value       10;
+            }
             implicit    0;
         }
 
         H2O
         {
-            explicit    1e-4; // kg/s
+            explicit
+            {
+                $squarePulse;
+                value       1e-4; // kg/s
+            }
             implicit    0;
         }
     }

tutorials/lagrangian/reactingParticleFoam/simplifiedSiwek/constant/fvOptions

@@ -21,10 +21,12 @@ source1
 
     timeStart       0.1;
     duration        0.4;
+
     selectionMode   cellSet;
     cellSet         ignitionCells;
 
     mode            uniform;
+
     temperature     2000;
 }
 

tutorials/multiphase/interFoam/laminar/wave/constant/fvOptions

@@ -32,9 +32,6 @@ option1
 
     value           (2 0 0);
     lambda          0.5;
-
-    timeStart       0;
-    duration        1e6;
 }
 
 

tutorials/multiphase/interFoam/laminar/wave/constant/fvOptions.verticalDamping

@@ -31,9 +31,6 @@ option1
     }
 
     lambda          0.5;
-
-    timeStart       0;
-    duration        1e6;
 }
 
 

tutorials/multiphase/multiphaseEulerFoam/laminar/injection/constant/fvOptions

@@ -21,9 +21,6 @@ options
     {
         type            massSource;
 
-        timeStart       0.1;
-        duration        5;
-
         selectionMode   points;
         points
         (

tutorials/multiphase/multiphaseEulerFoam/laminar/steamInjection/constant/fvOptions

@@ -21,9 +21,6 @@ options
     {
         type            massSource;
 
-        timeStart       1;
-        duration        500;
-
         selectionMode   points;
         points
         (