- mapFields and splitMeshRegions need more clarification

ENH: change some iterator -> const_iterator access

BUG: found some places with forAllIter and ::iterator !

- operator[] with label -> get args at index
- operator[] with word  -> get named option

- make centrally available via #include "addOverwriteOption.H"

- added directMapped*Patch constructors with uniform offset
- changed splitMeshRegions to use it
- adapted chtMultiRegionFoam tutorials

- ensure that the standard options (eg, from timeSelector) also have
  some usage information

- also drop various unused time options from src/OpenFOAM/include

- removed the previously added fileName type(), isDir(), isFile() methods.

- added optional bool parameter to isFile() function to explicitly
  enable/disable the check for gzip files.

- fixed minor bugginess where the default usage of isFile() would result in
  false positive matches.

- be slightly more stringent and use isDir() and isFile() instead of
  exists() function when we actually know the expected type.

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices