FixedMap and InverseMap

[dannys4]

Me and @rubiop seem to be tackling problems where we independently need functionality for two kinds of maps

• Fixing a slice of the input variables and pre-evaluating the corresponding functions, at least the beginning of the map (e.g. we have T(x,y) and we want the map T_y(x*,•), which is square)
• Taking the inverse of a map and using it as a map in and of itself (e.g. we have a map x = T(y) and we want the map y=T^{-1}(x) ). A lot of functions like Evaluate and Inverse are straight forward, Some functions like CoeffGrad are tricky, and some like SetCoeffs are extremely unclear.

[dannys4]

InverseMap is really hard-- the gradients become ridiculous (you end up having to do three-dimensional tensor contractions of derivatives with order >1)-- so we're definitely not adding that any time soon.