Complete intro_supply_demand.md

[jstac]

TODO -- edits

• Add some historical discussion of the fundamental welfare theorems
• wiki links for consumer surplus and producer surplus, or a slower explanation
• add figs to show the surpluses under different parameter values --- extract from https://github.com/QuantEcon/lecture-python-intro/blob/main/lectures/supply_demand_heterogeneity.md and https://github.com/QuantEcon/lecture-python-intro/blob/main/lectures/supply_demand_multiple_goods.md, simplify
• heading levels need to be consistent (see build error)
• full spell check
• headings need to be capitalized (to fit with other lectures)

TODO -- extension

Add the example from https://github.com/QuantEcon/cbc_workshops/blob/main/day_1/equilibrium.ipynb

• Illustrate surplus
• Compute surpluses using numerical integration
• Check welfare theorems numerically? As an exercise?

TODO --- code

If code is extracted from the other supply and demand lectures, check

• spaces around * and @ (closer to PEP8)
• camel case for classes, no underscore (consistent with PEP8)

[HengchengZhang]

Hi @jstac @mmcky, I'm looking at the issues with supply and demand lectures and is the building error you mentioned fixed in #143 by Matt?

[HengchengZhang]

And for the white spaces around @ and *, I think PEP8 doesn't have a clear rule:

• Always surround these binary operators with a single space on either side: assignment (=), augmented assignment (+=, -= etc.), comparisons (==, <, >, !=, <>, <=, >=, in, not in, is, is not), Booleans (and, or, not).
• If operators with different priorities are used, consider adding whitespace around the operators with the lowest priority(ies). Use your own judgment; however, never use more than one space, and always have the same amount of whitespace on both sides of a binary operator:

Shall we keep the white spaces in this case(like in other lectures)?

[jstac]

Yes, let's generally keep whitespaces around all operators except **

(Sometimes I break the rule if I'm trying to squeeze onto a line.)