Fix: thickness calving does not work with constant threshold in dev branch

[sschoell]

With the changes in the dev branch, it's not possible anymore to provide PISM with a constant threshold for thickness calving.

To Reproduce Running PISM with the command line argument pismr -i ../input/pism_equi_input.nc ... -calving eigen_calving,thickness_calving -thickness_calving_threshold 75.0 ... fails with the following error message, instead of taking the provided threshold:

* Initializing the 'calving at a threshold thickness' mechanism...
  - Option calving.thickness_calving.file is not set. Trying the input file '../input/pism_equi_input.nc'...
  Reading thickness calving threshold from file '../input/pism_equi_input.nc'...
PISM ERROR: can't find threshold used by the 'calving at threshold' calving method (thickness_calving_threshold) in ../input/pism_equi_input.nc.

Fix The condition in L73 doesn't work as expected, because opt.filename defaults to the input file if no calving.thickness_calving.file is provided. The proposed fix checks instead, if the variable exists in the file.

Checklist

• [ ] update documentation
• [ ] update CHANGES.rst
• [ ] add regression tests

[aaschwanden]

@sschoell : Thank you very much for reporting and fixing this bug!