Complete supply_demand_multiple_goods.md

[jstac]

Text to-do

• full spell check
• headings need to be capitalized (to fit with other lectures)
• search for ^T and replace with ^\top

Code to-do

• extract code from supply_demand_heterogeneity.md and specialize to the single agent case
• from the same lecture, shift over all examples that do not have heterogeneity
• spaces around * and @ (closer to PEP8)
• camel case for classes, no underscore (consistent with PEP8)