PEP8 Refactor NonlinearSolvers.py

Note: I left variable casing unchanged, since this is used consistently
elsewhere in Proteus.

Most of these changes do not affect functionality, however:

* I've changed at least one printed warning to an Exception
* I've removed dead code, and fixed a few broken imports/unresolved
  references
* I moved the csmoothers import to the top-level of the module.  Module
  imports do not belong in objects, since they are Singletons with
  global state, and likely to confuse developers.

This is a no-change refactor.  It should be reviewed by at least one of @cekees and @mfarthin, preferably both, before merge.