Add a masking functionality

[adehecq]

This is in reply to issue #235.
I added a new method to Raster to set the data mask.

Example:

import geoutils as gu
r = gu.Raster(gu.datasets.get_path(dataset))
mask = (r.data == 126)
r.set_mask(mask)
r.show(cmap='gray')

[rhugonnet]

Looks great! This will be super useful 👍. Doing this operation for masked arrays (especially within another object) is not something intuitive for all.
I have only a little comment on the tests: we could add an assertion using np.array_equal to check that the exact content of the mask is preserved? Right now if I got it right, it's only the counts.

[adehecq]

I have only a little comment on the tests: we could add an assertion using np.array_equal to check that the exact content of the mask is preserved? Right now if I got it right, it's only the counts.

Good point ! Especially when testing that the previous mask is preserved. Summing the count works if the two masks do not overlap (which is the case here), otherwise not...
Fixed with c2fe2a1.

[adehecq]

While I was on it, I also fixed issue #233.