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.
Another vestigial norm_r_last
Merge pull request #123 from erdc-cm/ahmadia/pep8_nonlinearsolvers

PEP8 Refactor NonlinearSolvers.py