Support NVHPC toolchain for PDAF

[jjokella]

Parent: #106

This PR aims to add changes into PDAF-build-scripts in order to support NVHPC.

For FOPT and COPT, copied flags from eCLM (HPSCTerrSys/eCLM#92)
Still missing (if needed):

• MKL linking flags
• double precision flags

[jjokella]

For eCLM-PDAF, I have the following error, already inside the PDAF-code, so it seems to compile quite far:

NVFORTRAN-S-0038-Symbol, isnan, has not been explicitly declared (eclm/enkf_clm_mod_5.F90: 625)
  0 inform,   0 warnings,   1 severes, 0 fatal for update_clm_swc

[jjokella]

With this setup, eCLM-PDAF with nvhpc runs the testcase clm5_pdaf_10x10 and gives typical compiler-change errors wrt to the reference for H2OSOI:

Max absolute difference: 0.00024265
Max relative difference: 0.00160455

Thus, I would say the PR is ready to go into the general nvhpc-branch