Disentangle derivative calculation and library

[GJBoth]

Now we calculate the derivatives and polynomials and construct the library in the same function. If we decouple this we can more easily use the library function outside of deepmod, i.e. to compare to classical algorithms. Thoughts?

[georgestod]

I like the idea, it will be easier to debug as well. As the problems become fancier, it might facilitate building custom libraries?