Make EnergyOutput wrapper class

Make EnergyOutput wrapper class

Eventually we want a proper class for the energy accumulation and
output behaviours. In the meantime, this hides much about t_mdebin and
t_ebin from integrators, so that we can refactor with minimal
disruption to other work.

Made t_mdebin private to the new EnergyOutput implementation file.
Used bool rather than gmx_bool in the interface of EnergyOutput.
Noted several TODOs for improvements to code structure.

Removed dependency on ebin.h and enxio.h from the new header, so
various other places had to declare their own dependencies.

Change-Id: If09b2c82dd8c139c76cb5fc453d556c26a5eee89