Improve expression error handling

[jtcooper10]

(See issue #687)

Improves the error handling of the Expression class, and allows for adding Python built-ins to be mapped to C++ ones.

Fixes

• Expression#getexpr_cpp and Expression#getexpr_python raise a SyntaxError on parsing validation errors, instead of returning None.
• Adds the Python abs function as a valid function identifier in Expression code.
  • Additional Python built-ins can be introduced by adding an entry to the Expression.function_map dictionary, where the key is the name of the Python function and the value is the C++ equivalent (they can, and often are, the same).

Closes #688