[WIP] Implement FIRE minimizer as CustomIntegrator

[jchodera]

This PR implements the FIRE minimizer (suggested in #85) from

Structural relaxation made simple.
PRL 97:170201, 2006
http://dx.doi.org/10.1103/PhysRevLett.97.170201
which uses a simple heuristic scheme that respects constraints.

[proteneer]

Interested to know if you guys are still working on this. I'd like to find a way to get rid of the conditional inertia check: P:=F.v < 0 to make it differentiable

[jchodera]

I think @hannahbrucemacdonald and @jaimergp were looking into this last.

Can't you just use a soft tanh-like function that scales the timestep by a continuously-variable amount depending on the sign and value of the momentum?

[proteneer]

I think that's definitely something we can look into, i.e. replacing the if/then (aka. heaviside function) with a tanh heaviside function.