v0.8.0

This is a major release that introduces some breaking changes.

The major improvement in this release will be to vertical methods. All vertical methods should now work with files with vertical axes that are either consistent or vary spatially. Before some methods only worked with z-levels, i.e. files with fixed vertical levels.

This change will result in a requirement that vertical_mean, vertical_interp and vertical_integration need users to specify whether the vertical levels are fixed spatially, using the fixed arg. As a result, calls to these methods without this argument will throw an error.