gmxapi.mdrun has not guaranteed "trajectory" output

The output.trajectory attribute of gmxapi.mdrun references was hard-coded to look for traj.trr in the resolution to #3144 (closed).

However, this file is not guaranteed to exist.

The proposed fix is to

1. map the trajectory output to the value of the -o mdrun argument.
2. guarantee the presence of a -o mdrun argument (with a default value of traj.trr.

This issue will be resolved as part of #4284 (closed).