Create an optimization function which processes the graph and convert…

[tarkmeper]

This pull request isn't in any way ready for merging but is more of an idea I'd like to get feedback on and if this is something you'd consider for inclusion in Pycel before putting in more effort to get it into that shape.

The code creates an optimized version of the logic based on the desired inputs/outputs. It traverses the graph and merges all the python_code text into a single function that is dynamically created and returned. Very simple functions can also be optimized with Numba to allow for compiler optimizations.

On my computer on a simple test case:
* Calling "evaluate" directly on outputs took 11.14 seconds
* Using non-Numba optimized version took 29ms (381x speed-up)
* Using numba optimized version took 0.5 ms (2228x speed-up)

Todos:
* Some code clean-up and comment cleanup
* Import remaining functions - only sin/sum currently will work.
* Loads more testing
* Better API for generated function currently takes and returns tuples in order to allow numba support.

[dgorissen]

Very cool. Looks useful to me though I no longer have the bandwidth to actively review and test pycel contributions in detail.
@stephenrauch has been closer to the repo and logic so can let him wade in.

[amankagarwal]

Looks very cool @tarkmeper - was wondering if you ever got this working end-to-end, would love to try it out!