Support for generating polarised sky simulations.

* Extended `galaxy.ConstrainedGalaxy` with a simple polarised model.
* Added polarisation generation to point source classes.
* Extended `maps.Sky3D` with a `getpolsky` routine that simply returns Q=U=0 (used for 21cm signal).
* Added a `--pol` option to the `mk_sky*.py` scripts to force inclusion of polarisation.
* Additionally there is a fix for skysim.clarray when passing a z array which doesn't strictly increase.
Merge branch 'master' of github.com:jrs65/simulations_21cm

Conflicts:
	scripts/mk_sky_foregrounds.py