Include ability to set/get Lookups

[pbreach]

This would be useful in VensimPLE as there seems to be no good way of specifying exogenous data other than using a lookup with Time variable as input. Being able to easily reset would mean that multiple runs could be made with different sets of exogenous input.

Vensim's setval command already includes the ability to reset lookup tables using (x,y) pairs:

SIMULATE>SETVAL|mylookup((0,0),(1,1),(2,2),(5,5))

Input could be specified as a dict where the pairs could be keys and values. Otherwise a list of tuples could work and might be more intuitive.