Allow user input sort functions in ImageDict.sort_keys()

[melissaebrooks]

The sort functions in an ImageDict are currently limited to a set of functions that sort strings that are relevant to meteorology and oceanography. The library was designed to handle large numbers of images produced when monitoring the output of Nnmerical Weather Prediction (NWP) models, or in evaluating changes to NWP models.

This is a limitation that would be experienced by users outside of meteorology, that would be alleviated by allowing the user to input their own sort functions.