Alternate ways to pass in velocities for OTF

[stevetorr]

It would be nice if we could pass in initial velocities to OTF besides the slightly awkward ( :) ) prev_positions argument.

A few possible ways forward that jump out at me:

1. One could re-factor the OTF module to accept a FLARE structure as input which the user could decorate with velocities.
2. Accept velocities as an argument to OTF.
3. Accept a temperature as an argument, which can be used to draw from a Maxwell-Boltzmann distribution to start off the run.
4. A 'back-propagation' convenience method on the FLARE structure or in util which could generate the previous positions from a given velocity (I don't like this one, for reference, but just throwing it out there).