Product Space calculations

Done:

• rca (and fair share)
• proximity
• density
• distance
• complexity (returns geo and product complexity)

TODO:

• opportunity gain (requires pci from complexity (product complexity))
• mhat (requires stats module)
• rca confidence (requires stats module)

Future work:

• better docs for calculations, relate the paper and formulas to the code
• serializing to json (maybe csv)
• create workspace
• product space calc server