Support for simulating real point sources.

 - Added catalogue of point source data from NVSS and VLSS, plus code
   for generating it.
 - Added `pointsource.RealPointSources` class for generating sky
   simulations.
 - Modified `pointsource.CombinedPointSources` to add in bright real
   pointsources.
 - This means that the `cora-makesky driver` should support them out
   of the box.
Merge pull request #9 from jrs65/real-point-sources

Add Realistic Point Source Model