Add/Test flags forcing implicit none

[mathomp4]

Per @tclune, many out there in Fortran land use compiler flags to enforce implicit none by default. From what I can gather from compilers I have access to, the flags are:

• Intel: -implicitnone (hat tip @milancurcic via a blog post, as the manpages were not too helpful)
• GNU: -fimplicit-none
• NAG: -u
• NVHPC: -Mdclchk

Obviously, all GEOS/MAPL code should use implicit none but, well, GEOS has some ancient code and this would at least let us find those stragglers and prevent newer Fortran coders wondering "How does this loop work since i was never declared!"