Extending PPMPF_VXPP_FMAPOF with PPMPF_VXPP_FMAPSETS macro

Now we can expand multiple ppmpf macro set identifiers in one go, provided
PPMPF_VXPP_SETS() is be defined as a 0- to 10-tuple of set macro
identifiers (PPMPF_VXPP_SET0,...9) with only the identifier digit.
An example follows:

    #define PPMPF_VXPP_SET0(n) ((f),(0,100))
    #define PPMPF_VXPP_SET1(n) ((g),(10,50))
    #define PPMPF_VXPP_SETS()  (0,1)
    #include PPMPF_VXPP_FMAPSETS() // will expand 0 then 1 etc.

This way we do not need multiple PPMPF_VXPP_FMAPOF inclusion directives
since everything is handled by a single 'inclusion function metamacro'.

Some documentation added for this as well.
Added PPMPF_VXPP_ISSET as a set macro identifier checker.

If the parenthesis enclosed comma separated list of 0-9 digits in the
PPMPF_VXPP_SETS() invocation is invalid, this will signal an #error
directive and prevent expansion from occuring.