Non-compliance with Fortran 95 standard

In src/frun_GR6J.f90 file, lines 299 to 312 are not compliant with Fortran 95 standard, because of the GOTO instruction. Te GOTO feature was considered obsolescent in Fortran 90 and it was recommanded to replace it by a CASE or an IF ELSE construct.

Here attached is a proposed construct that is compliant with Fortran 95 standard.frun_GR6J.f90