Add checks for gradient completeness/correctness

[sethaxen]

As raised in salilab/imp#1020, IMP currently silently fails when a derivative is requested from a non-differentiable restraint. That's not good in general, but that's catastrophic for HMC. In addition to the check in #1, we should add a check that uses central difference approximations to validate the gradient at the current model configuration. It's ad hoc and local but should identify if there are any major issues with the gradient.

This and #1 can be bundled into a single function that locally validates that things are working correctly for IMP.hmc and that the user would be recommended to run on a new model to diagnose issues before using IMP.hmc.