Add support for morphing formulae

[JaySandesara]

Currently the non-constrained parameters entering the model are only allowed to have a linear scaling behavious with normfactor, following canon pyhf serialization format.

Enhancement: add support for users entering a morphing formula or nodes/templates to be fit using a polynomial formula.

Example use cases: HH, off-shell Higgs, EFT, etc.

[Ragansu]

A potential solution to this is addressed in PR #60 which solves system of equation to generate a Vandermonde Matrix, The update has been testest for stability.