Develop branch does not compile with NAG

Having set the newest NAG 6.2 compiler on EVE, we can now test mHM again. When doing: make compiler=nag system=eve release=debug -j 4 we get the following:

Questionable: /gpfs0/home/ottor/lib/mhm/src/mRM/mo_mrm_river_head.f90, 
line 162: Variable NCOLS0 set but never referenced                     
Questionable: /gpfs0/home/ottor/lib/mhm/src/mRM/mo_mrm_river_head.f90, 
line 162: Variable NROWS0 set but never referenced                     
Error: /gpfs0/home/ottor/lib/mhm/src/mRM/mo_mrm_river_head.f90, line 31
7: Implicit type for ISNAN in CALC_SLOPE                               
[NAG Fortran Compiler pass 1 error termination, 1 error, 2 warnings]

I assign @schueler as he is creator of the file