Solve typing error in destriper

[nraffuzz]

This PR solves the issue #270.

[nraffuzz]

Since weights should be an array (field initialized by get_map_making_weights), also obs.net_ukrts should be an array.
In order to handle this I added:

if isinstance(obs.net_ukrts, (float, int)):
    obs.net_ukrts = obs.net_ukrts * np.ones(obs.n_detectors)

inside get_map_making_weights.

I think this PR could be closed.

[ziotom78]

Looks ok to me. Feel free to update the CHANGELOG and merge this PR!