code cleanup

[jeremymanning]

Note: Upon further reflection, I don't think it makes sense to force timecorr to return TimeCrystals. I think TimeCrystals should be an internal class type, specifically used for our synthetic data explorations (at least until we have a compelling reason for why users would want to use them). I've therefore:

• reverted back some of the changes to timecorr and levelup, so that they now return numpy arrays or lists of arrays (as they used to)
• added a format_data function in helpers that wraps hyp.tools.format_data but also adds support for TimeCrystal objects. that's now used preferentially over hyp.tools.format_data within the main timecorr function.

[jeremymanning]

I also changed some syntax to make it cleaner, but in turn I broke support for Python 2.7. However, I think we're going to be moving away from supporting 2.7 anyway soon (since I don't think numpy and pandas will continue to support it, and we rely on those packages for timecorr). So I've removed support for 2.7. The officially supported version of python is now 3.6+.