Add helper for generic C++ command-line modules

Add a helper class (gmx::CommandLineOptionsModuleInterface) that makes
it easier to implement proper C++ command-line modules for the wrapper
binary.  The module only needs to implement the interface instead of
gmx::CommandLineModuleInterface, and the helper class then takes care of
command-line parsing and help, based on options initialized by the
module in initOptions().  This removes the need to duplicate
command-line parsing and help code into each module.

Change-Id: I3e213ce530532bd3f60e34fd261fe06797d10326