Project on subspace operator

[adler-j]

When reading the discussion in #49, I realized that we could use a "project on subspace" operator

class Projection(odl.Operator):
    def __init__(self, space, indices):
        assert isinstance(space, ProductSpace)
        self.indices = indices
        super().__init__(space, space[indices], linear=True)

    def _call(self, x):
        return x[self.indices]

[kohr-h]

True, maybe call it ComponentProjection?

[adler-j]

Yes that is possibly a better name!