Support assignation for types other than CubePrimitive

[maxtremblay]

Currently, we can only do the following when value has type CubePritimive.

value = some_function(/* ... */);

However, for many types, for example a tuple where are elements are CubePrimitive it should be easy to derive an assignment implementation that works elementwise.

I suggest introducing a new trait CubeAssign with a possible derive to support that feature for most CubeType.