Ensure latitudes decreasing in normalize_dataset

[pont-us]

As described in the related issue #251 , the xcube viewer currently requires latitudes to be in descending order. Issue #251 is the issue for actually fixing this in the viewer (since both latitude orderings should be supported in xcubes). This issue is for the temporary fix of reordering the latitudes.

Problem: datasets with ascending latitudes don't plot in the xcube viewer.

Suggested (temporary) solution: make normalize_dataset in core.store.normalize ensure ascending latitudes. Currently it does the opposite, ensuring descending latitudes. This will not automatically ensure descending latitudes in every cube, but will force them to descending when normalize_dataset is called from a plugin to carry out the standard normalizations on an imported dataset.