Standardise numpy namespace. Closes #210.

Changes all numpy imports to the form `import numpy as np`.

This PR does not change any existing use of the `numpy.ma` module as I believe there is some disagreement about how/if these imports/usages should be standardised @pelson @rhattersley?

It might be worth noting that `geoaxes.py` has mixed use of `import numpy.ma as ma`/`ma....` and `import numpy as np`/`np.ma....`. These cases are isolated to the `_pcolormesh_patched` and `_pcolor_patched` work-around methods which `import numpy.ma as ma` locally, but sometimes also use the globally imported name `np.ma`...

Anyway, let me know what you think.