ENH: simplify check for metis sizes (issue #290)

- Now that the metisDecomp uses the metis definition for float/double,
  do not need to verify the scalar sizes.

Note:
- could drop precision qualifier for metis, scotch installation
  (include, lib) as being unneeded, but it is simpler to keep them
  and continue to use the FOAM_EXT_LIBBIN path
  (in case other ThirdParty software is compiled with different
  precisions).