Implement FIND MARKET PRICE and SUPPLY AT PRICE #638
Description
These Vensim functions are needed for EPS. Implement them in the C runtime and add model tests.
The Vensim docs note that these are similar to the ALLOCATE AVAILABLE function. While we are implementing them, we should also revisit the convergence criterion for ALLOCATE AVAILABLE. It is currently set to a small absolute difference, but this may incorrectly show that convergence is failing when the magnitude of the values is large enough.