Renames and small changes in analysis._interpolator, to improve code clarity.

This started out as a quick look into instance-variable names.
Although that had already been greatly improved since the task was raised, I thought it might still be worth kicking the tyres. 

I got a bit carried away, and there is rather more to it now...
In the process of sorting out in my head the functions of the various instance variables, I realised that the pre-processing of the original cube data in "_extend_circular_coord_and_data" appeared to be doing nothing at all.  It turned out is was possible to avoid that, remove the extra method and --big bonus-- stop it from loading the source-cube data.

That's what is here, along with another couple of refactors to improve code clarity a bit.

Probably best to treat this as a series of suggestions.
  * I'll add some notes on the diffs
  * individual changes not liked can be reverted if necessary.