Add overloading methods to Raster

[atedstone]

We would like to be able to support operations like this:

im1 = Raster(...)
im2 = Raster(...)
combined = im1 + im2

At the moment, this requires a fairly unsemantic approach:

combined = im1.copy()
combined.data = combined.data + im2.data

Proposal: add operator overloading methods to the Raster class, e.g. __add__, __multiply__. See e.g. https://stackabuse.com/overloading-functions-and-operators-in-python/ for more info.

[adehecq]

Now done for add/sub and neg with PR #173.
Still waiting to add mult/div to close this issue.

[adehecq]

See https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types for a list of arithmetic operations that can be overloaded, as well as for the difference between e.g __add__ and __radd__.

[adehecq]

Closed via #257