Structure of Python cheatsheet

[jstac]

The content needs to be expanded and the structure could be improved. For example, if/else comes under the heading Functions.

I suggest we start with a headings like these:

Operators (covers *, +, **, etc.) Data Types Builtin Functions Imports Input and Output Iterating Comparisons and Logical Operators User Defined Functions

Then we can move on to NumPy and plotting.

[vgregory757]

These categories are much better! I started off by rearranging what we have now: https://github.com/QuantEcon/QuantEcon.cheatsheet/commit/32369c20e91c9e6bc253ea7e38658b3d36eb0e82

[jstac]

@vgregory757 Great, thanks!