Python, separate file handling

[jonmoore]

File handling is via Python's 'import' mechanism. Each Python file is effectively treated in the same manner as external python libraries. The TI Inspire functions in the same way, as do Casio calculators.

Round trips via PPL appear to be less performant than using the numerical and symbolic views of the Python app itself. The numerical view is the Python terminal and the symbolic view is the Python text editor.