Add force correlation to AWH module

Add force correlation to AWH module

This change adds the calculation of force correlation statistics
during an AWH biased simulation.
The main quantity of interest is the time-integrated force
correlation, also known as the friction tensor
(see e.g. http://dx.doi.org/10.1103/PhysRevLett.108.190602).
The friction tensor defines a metric on the coordinate space and
the local volume element of this metric is a useful measure for
determining which regions need more or less sampling.
gmx awh prints the friction (tensor) and can also still process
energy files without friction data.

Change-Id: I164be4665004dea5b250e3c7ac135ac1c1cbd783