Add `LOOKUP INVERT`

[JamesPHoughton]

LOOKUP INVERT takes a lookup function (not a call to it) as a parameter, along with a y value on that table, and returns the x value at which the table should return y.

It's useful for creating expressions to initialize a model in equilibrium when that equilibrium depends on a value in the table.

See the vensim docs

This may require some changes to the parser to recognize the reference to the lookup table which is not actually a call to the lookup table.

[JamesPHoughton]

This can't be just a straight up optimization on the function call, as the lookup table could have flat regions which throw off the lookup. Needs to actually identify the points that it should be looking at, and then run an interpolation.

This may suggest making lookup tables objects containing a pandas series, and then making several operations on that series possible. That would also allow other types of lookup table calls, such as LOOKUP BACKWARD and LOOKUP AREA.