ENH: Unit Test FunctionParser

A unit test to cover almost 100% of a critical module.
With current patch, 95% coverage.

While building this unit test, repaired and enhanced:
vtkFunctionParser
  1) More consistent error messages
  2) Pow error checking wrong
  3) Removed unreachable code
vtkTestErrorObserver
  1) Accumulates error/warning messages until cleared. Prior behaviour
     was to only report last error/warning.

Change-Id: I62d011fcda5a097943553316b68334c406b87cfe
Merge topic 'UnitTestFunctionParser' into master

5f3f8374 ENH: Unit Test FunctionParser