Debug and update print_update_clm netCDF output #76

During work on

• PDAF-OMI for GRACE-DA and SM-DA #26

it was found that the print_update_clm leads to silently stopping
simulations, whenever CLM:update_swc and CLM:print_swc are
switched on in enkfpf.par.

This should be addressed, at the very least giving a clear warning /
error message, but best by updating the printing subroutine.