ENH: argList::getList with optional parameter

- this allows it to work like readListIfPresent() but with a list as
  the return value, which can be useful for a const context.

applications/utilities/postProcessing/miscellaneous/temporalInterpolate/temporalInterpolate.C

@@ -215,8 +215,8 @@ int main(int argc, char *argv[])
     #include "setRootCase.H"
     #include "createTime.H"
 
-    wordRes selectedFields;
-    args.readListIfPresent<wordRe>("fields", selectedFields);
+    // Non-mandatory
+    const wordRes selectedFields(args.getList<wordRe>("fields", false));
 
     if (selectedFields.empty())
     {

applications/utilities/preProcessing/mapFieldsPar/mapFieldsPar.C

@@ -288,8 +288,8 @@ int main(int argc, char *argv[])
         Info<< "Subtracting mapped source field from target" << endl;
     }
 
-    wordRes selectedFields;
-    args.readListIfPresent<wordRe>("fields", selectedFields);
+    // Non-mandatory
+    const wordRes selectedFields(args.getList<wordRe>("fields", false));
 
     const bool noLagrangian = args.found("noLagrangian");
 

src/OpenFOAM/global/argList/argList.H

@@ -371,12 +371,15 @@ public:
             const T& deflt
         ) const;
 
-        //- Read a List of values from the named option,
+        //- Get a List of values from the named option,
         //- treating a single entry like a list of size 1.
+        //  \param optName  the option name to read from
+        //  \param mandatory if the option is non-mandatory, the behaviour
+        //      is similar to readListIfPresent().
         template<class T>
-        inline List<T> getList(const word& optName) const;
+        inline List<T> getList(const word& optName, bool mandatory=true) const;
 
-        //- If named option is present, read a List of values
+        //- If named option is present, get a List of values
         //- treating a single entry like a list of size 1.
         //  \return true if the named option was found.
         template<class T>

src/OpenFOAM/global/argList/argListI.H

@@ -322,14 +322,22 @@ inline Foam::List<T> Foam::argList::getList(const label index) const
 
 
 template<class T>
-inline Foam::List<T> Foam::argList::getList(const word& optName) const
+inline Foam::List<T> Foam::argList::getList
+(
+    const word& optName,
+    bool mandatory
+) const
 {
-    ITstream is(optName, options_[optName]);
-
     List<T> list;
-    readList(is, list);
 
-    checkITstream(is, optName);
+    if (mandatory || found(optName))
+    {
+        ITstream is(optName, options_[optName]);
+
+        readList(is, list);
+
+        checkITstream(is, optName);
+    }
 
     return list;
 }
@@ -345,6 +353,7 @@ inline bool Foam::argList::readListIfPresent
     if (found(optName))
     {
         ITstream is(optName, options_[optName]);
+
         readList(is, list);
 
         checkITstream(is, optName);