ENH: add single-time handling to timeSelector

- the timeSelector is often used to select single or multiple times
  (eg, for post-processing). However, there are a few applications
  where only a *single* time should be selected and set.

  These are now covered by this type of use:

      timeSelector::addOptions_singleTime();  // Single-time options
      ...
      // Allow override of time from specified time options, or no-op
      timeSelector::setTimeIfPresent(runTime, args);

   In some cases, if can be desirable to force starting from the
   initial Time=0 when no time options have been specified:

      // Set time from specified time options, or force start from Time=0
      timeSelector::setTimeIfPresent(runTime, args, true);

   These changes make a number of includes redundant:

     * addTimeOptions.H
     * checkConstantOption.H
     * checkTimeOption.H
     * checkTimeOptions.H
     * checkTimeOptionsNoConstant.H

ENH: add time handling to setFields, setAlphaField (#3143)

    Co-authored-by: Johan Roenby <>

STYLE: replace instant("constant") with instant(0, "constant")

- avoids relying on atof parse behaviour returning zero

tutorials/incompressible/lumpedPointMotion/bridge/code/polynomial-motion.C

@@ -534,7 +534,7 @@ int main(int argc, char *argv[])
 
     #include "createTime.H"
 
-    runTime.setTime(instant(runTime.constant()), 0);
+    runTime.setTime(instant(0, runTime.constant()), 0);
 
     #include "createNamedMesh.H"