ENH: add 'default' as possible Switch state, but not as input/output

- in some circumstances we need to pass a bool value upwards to the
  caller and know if the true/false value was set based on real input
  or is a default value.

  Eg, in the object::read() we might normally have

     enabled_(dict.readIfPresent(key, true));

  but would lose information about why the value is true/false.

  We can change that by using

     enabled_(dict.readIfPresent<Switch>(key, Switch::DEFAULT_ON));

  After which we can use this information is testing.

      if
      (
          child.enabled().nonDefault()
        ? child.enabled()
        : parent.enabled()
      )
      { ... }

   And thus enable output if the parent requested it explicitly or by
   default and it has not been explicitly disabled in the child.

  No difference when testing as a bool and the text representation
  of DEFAULT_ON / DEFAULT_OFF will simply be "true" / "false".

ENH: add construction of Switch from dictionary (similar to Enum)

applications/test/primitives/Test-primitives.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+    \\  /    A nd           | Copyright (C) 2017-2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -35,6 +35,8 @@ Description
 #include "NASCore.H"
 #include "parsing.H"
 #include "Tuple2.H"
+#include "Switch.H"
+#include "dictionary.H"
 
 using namespace Foam;
 
@@ -45,6 +47,20 @@ inline scalar readNasScalar(const std::string& str)
 }
 
 
+// As a function
+inline Switch readSwitch(const std::string& str)
+{
+    Switch sw(str);
+
+    if (sw.type() == Switch::ON)
+    {
+        Info<< "Was 'on'" << nl;
+    }
+
+    return sw;
+}
+
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 template<class T>
@@ -96,7 +112,8 @@ unsigned testParsing
     string errMsg;
 
     // Expect some failures
-    const bool prev = FatalIOError.throwExceptions();
+    const bool prev1 = FatalError.throwExceptions();
+    const bool prev2 = FatalIOError.throwExceptions();
 
     for (const std::pair<bool, std::string>& test : tests)
     {
@@ -120,7 +137,8 @@ unsigned testParsing
         hadParsingError(test, result, errMsg);
     }
 
-    FatalIOError.throwExceptions(prev);
+    FatalError.throwExceptions(prev1);
+    FatalIOError.throwExceptions(prev2);
 
     return nFail;
 }
@@ -130,6 +148,36 @@ int main(int argc, char *argv[])
 {
     unsigned nFail = 0;
 
+    {
+        Info<< nl << "Test Switch parsing:" << nl;
+        nFail += testParsing
+        (
+            &readSwitch,
+            {
+                { false, "True" },
+                { true,  "false" },
+                { true,  "on" },
+                { false, "None" },
+                { false, "default" },
+            }
+        );
+
+        dictionary dict;
+        dict.add("key1" , "true");
+
+        {
+            Switch sw("key", dict, Switch::DEFAULT_ON);
+            Info<<"got: " << sw << " type is DEFAULT_ON? "
+                << (sw.type() == Switch::DEFAULT_ON) << nl;
+        }
+
+        {
+            Switch sw("key1", dict, Switch::DEFAULT_ON);
+            Info<<"got: " << sw << " type is DEFAULT_ON? "
+                << (sw.type() == Switch::DEFAULT_ON) << nl;
+        }
+    }
+
     {
         Info<< nl << "Test readDouble: (small=" << doubleScalarVSMALL
             << " great=" << doubleScalarVSMALL << "):" << nl;