pmlmodelling/nctoolkit: v0.3.8

This is a minor release, focusing on under-the-hood improvements and introducing better handling of files with varying vertical layers.

A method, vertical_integration for calculating vertically integrated totals for netCDF data of the likes of oceanic data, where the vertical levels vary spatially, is introduced. vertical_mean has been improved and can now calculate vertical mean in cases where the cell thickness varies in space.

merge_time is deprecated, and its functionality will be incorporated into merge. So, following this release ensemble merging operations should use merge.

open_url is now able to handle multiple urls. Previously it could only handle one.

Some under-the-hood improvements have been made to assign to ensure that truth statements do not occassionally throw an error.

Full Changelog: https://github.com/pmlmodelling/nctoolkit/compare/v0.3.7...v0.3.8