density fitting forces

density fitting forces

Evaluate forces on atoms that increase a measure of the goodness of fit to a reference density.

The density fitting forces are evaluated as the gradient of the goodness of fit of a comparison density is generated from atoms by spreading on a grid via a Gaussian kernel to the reference density.

Applying the chain rule for this gradient yields the density fitting force as the inner product of the derivative of the comparison density to reference density goodness-of-fit function at all density values and the gradient of the spreading kernel used to simulate the density.

This patch assumes that the comparison density to reference density goodness of fit derivative density is given, methods that aid the evaluation of this derivative density are part of a child patch.

refs #2282

Change-Id: I4d93fffbdb41753f9a0a4af921011aca02ade69c