Add ternary operation in stockflow macro syntax

[neonWhiteout]

Allows use of the syntax A = B ? C : D in a dynamic variable, where B, C and D can be symbols or expressions. An expression of this form is converted into a function call cond(B,C,D) = B ? C : D

[neonWhiteout]

Probably wanna try actually running the differential equations on one of these. If you have an example which uses an if else in another software, try porting it.

[Saityi]

The changes look good to me, notwithstanding the current failing notebooks validation.

We should probably make sure the docstrings at https://github.com/AlgebraicJulia/StockFlow.jl/pull/135/files#diff-4e1b9ec77867b43e29dfce6850ad2d9cab1373bef6eed4a38ac4b5c29f94c2a7L5 and https://github.com/AlgebraicJulia/StockFlow.jl/pull/135/files#diff-4e1b9ec77867b43e29dfce6850ad2d9cab1373bef6eed4a38ac4b5c29f94c2a7L116 are kept up-to-date

(I.e., the module-level docstring and the docstring on the stock_and_flow macro.

[neonWhiteout]

@Saityi I did a few changes so || and && can be used as well. Just convert A || B to or(A, B) := A || B and A && B to and(A,B) := A && B

[neonWhiteout]

@Saityi I did a few changes so || and && can be used as well. Just convert A || B to or(A, B) := A || B and A && B to and(A,B) := A && B

...Which is stupid, because || and && only allow bools. I guess we do:
or(A, B) := int(Bool(A) || Bool(B))

...Eh, guess we don't need that outer int wrap

[neonWhiteout]

...No, no, it might still work, since A < B should return a bool