BUG: Raise a quieter `MaskedArrayFutureWarning` for mask changes.
BUG: Raise a quieter `MaskedArrayFutureWarning` for mask changes.

Drop the `__getitem__` warning. In `__setitem__` check to see if the
masked array is shared. If it is shared, we know it will propagate
upstream in the future. Also, use a more specific warning type instead
of using `FutureWarning` so that this can be explicitly ignored.

ESSS/numpy,ESSS/numpy,ESSS/numpy