Add AWH biasing module + tests

Add AWH biasing module + tests

The AWH (Accelerated weight histogram) method is an adaptive biasing
method used for overcoming free energy barriers and calculating
free energies (see http://dx.doi.org/10.1063/1.4890371). Although
AWH can in general bias any system parameter, this change only
implements biasing of reaction coordinates. The actual force
distribution and coordinate handling is taken care of by the pull
code. AWH interacts with the pull code by registering itself as
the external potential module for the coordinate that should be
AWH biased. The AWH code sets the potential and force for those
coordinates.

The Grid test checks that the neighborhoods are correct.
The Bias tests check the force, bias and free energy values
for the final and initial phase, with MC and convolved force
and without and with skipping updates.

Change-Id: I202f58f7042e8e63c9d708fdcaca6da7e8a4022e