Add gmx convert-trj

Add gmx convert-trj

Adds a new module gmx convert-trj, aimed at providing the minimal
functionality to convert different GROMACS supported trajectory formats
into each other, as well as supporting selections to chose atoms for
writing to disk.

The tool is based on the OutputManager and OutputAdapter framework for
writing new trajectory files and for setting meta information in the
coordinate datastructures.

Refs #2115, #2396

Change-Id: I1413721faa1383a9d3b8f14cdbea2bcc3089273f