Small cleanup of sampler interface

Created by: PhilipVinc

this is the first commit of #707. It moves the function that gets jit-compiled to a function so we recompile less when creating new samplers. This was totally uninteded and probably an overlook on my part.

It also fixes an issue: in theory __repr__ should not be used to pretty print objects, but it should output parseable code. So samplers should not overload this for pretty printing. So i implement ipython pretty print in place of it.