nctoolkit: A Python package for netCDF analysis and post-processing

This is a minor release that makes some internal improvements to methods.

The vertical_mean method has been tweaked so that it will fix any inconsistencies between the data and the thickness file. Total vertical thickness, i.e. depth, is now only calculated based on cells with both data and a thickness; whereas before it was assumed that the thickness file's vertical description, and missing values, were consistent with the data. Cases where thicknesses are misleadingly provided are now dealt with by ignoring the thickness.