Avoid FP exception with empty .mdp file

Avoid FP exception with empty .mdp file

The default value for nsteps is zero, which used to lead to division
by zero in the above case. Refactored to avoid that.

Added a basic test case for grompp with an empty .mdp file (which
causes all default settings to be used). It is not optimal that the
mdrun integration test machinery is recycled for this, but
re-implementing or refactoring that to avoid the ugliness is not great
for a release branch.

Noted some TODOs. Renamed some variables for clarity.

Change-Id: I5c98c4f460f658277490b11f7fb5629d6fa84a08