Added support for polarised sky simulations.

Galaxy:
 - Generate polarised simulations based on Faraday rotation measure
   synthesis.
 - Tweaked unpolarised generation to better match real skies by
   reducing smoothing of Haslam map.

Point sources:
 - Generate random polarised point sources by assigning a random
   polarisation angle, and amplitude to each sky pixel and multiplying
   with unpolarised sky.
 - Routine for Faraday rotating sky.
Merge pull request #3 from jrs65/polarised-foregrounds

Polarised foreground simulations.