LAMA to Dask: `_combined_units` & further branch tidy

[sadielbartholomew]

Ultimately reinstate a commented-out test case, failing due to a minor issue in _combined_units, which turned out to simply be a use of the property Data._size which has straightforwardly been replaced by Data.size. I also updated all other remaining usage of _size in the Data module, which turned out to be two extra cases overall.

There were in fact two test cases due to be reinstated from the above fix, but one (that concerns __ipow__) is failing due to data type differences summarised by #435, so I have updated the messages provided by the commented-out case.