sympy_parser: don't change attribute access into Symbols (#2663)

Only fixes the issue relating to attribute access, not keyword
arguments.
sympy_parser: don't change keyword arguments into Symbols

Fixes #3423 and #2663.
sympy_parser: Add tests for keyword args and attributes
Refactor sympy_parser
sympy_parser: Update tests to reflect new API
parse_expr: Allow override of globals
Add imports for tests
sympy_parser: improve documentation
Revert to old transformation API
Fix bug with _kern
sympy_parser: update docs
sympy_parser: update doctests
Change parse_expr API to be backwards-compatible
sympify: Include original exception in SympifyError
Python 2.5 compatibility
Binomial coeff are not expanded by default
Made some fixes, updated some tests
Added functions for dnf format, added simplification tweak
Fixed doctest. Added tests, documentation and doctest
Condensed common code