ENH: Add GENERATE_TCLAP_ASSIGNMENT_IFSET generation.

The GENERATE_TCLAP macro is split into GENERATE_TCLAP_PARSE
and GENERATE_TCLAP_ASSIGNMENT.  Another macro is also created
that assigns the C++ variables only if they have been set on the command line.

Currently, GENERATE_TCLAP_ASSIGNMENT is not used by PARSE_ARGS, but it is used
by TubeTK to overwrite parameters that have not been set.