ENH: Use an output formatter

[HaoZeke]

Split from #155.

In any case, for the kind of data we store, HDF5 is probably the best bet (maybe via highfive or h5cpp), corresponding to one data point (multi dimensional) with energy, variance (single values) and forces (multi dimensional).

This is separate from the .con files for path tracing, and more of an issue for the current .dat results.

[HaoZeke]

One thing about the C++ helpers here is that they have non-MIT licenses, but that should be OK..