Create separate module for trajectory data

Create separate module for trajectory data

Move trx.h from fileio/ to a new trajectory/ module, and rename it to be
more descriptive.  This directory can then evolve to have the basic data
structures and operations on trajectory information, similarly to the
topology module for structure information.  This removes a
pbcutil <-> fileio cyclic dependency, and also makes the selection code
independent of fileio.

Moved energy.h from mdtypes/ to the same place, since it is used for
representing energy information during the trajectory.  This makes
mdtypes at least slightly more mdrun-specific.

Part of #1854.

Change-Id: I4c266d78daefc3be9adefb56cd7cce8d9548634b