Simple way to get last frame from trajectory - Redmine #2873

At https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-users/2019-February/124407.html, a user asked for a simple way to get the last frame from a trajectory.

Currently gmx trjconv -f -dump 9999999 doesn’t work because an arbitrary time is observed to be inconsistent with the time series.

We could implement gmx dump -f traj.tng -lastframe conf.gro

(from redmine: issue id 2873, created on 2019-02-27 by mark.j.abraham)