Generalize time post-processing

[darothen]

Trying to use this package wth Fernando's large ensemble fails because he didn't include traditional time indices along his post-processed output - he simply has a "date" dimension, which is a YYYYMMDD time as an integer.

All of the date processing logic can actually probably occur as a pre-processing function passed by the user. But a more general abstraction for this functionality - metadata post-processing in general - would be useful.