Use scipy solvers for finding SDF and correction factors

[jo-tham]

Essentially, the sdf functions solve the desntity functions for SDF given the
observed age, site index etc

if we use a function to generate the density functions such that they are only variable in SDF, a scipy solver can be used

this reduces our maintenance overhead and probably gives a speed improvement

This is likely also true for the basal area correction factor functions of which the aw function is currently the bottleneck in gypsy