Fortran: check for undeclared variables by forcing implicit none everywhere
C/C++: check for unused parameters and return types
removed unused variables from test cases
ci: do missing return and unused arg check with Github Actions
Apparently we have no tests for this because this is broken pretty
badly. This extends the basic test to actually check for the correct
free-form argument and thus test this.
And fix the list of supported file suffixes, and use .f90 for all
fortran tests since ifort, the Intel Fortran compiler ignores files
ending with .f95, .f03, and .f08
Rohit Goswami