implement prior generation
This MR implements parsing a mixture of specified priors, fixed parameters and then adding in defaults.
There's a new function in prior.py and a couple of changes to run_sampler.py.
An example of the new usage is shown in BasicTutorial.py and Injection.py.
Thoughts @paul-lasky @gregory.ashton @moritz.huebner?
Edited by Colm Talbot