Rewrite ACE.jl as a Graph

[cortner]

In order to properly leverage the AD eco-system we will need to rewrite ACE.jl as a graph. I see no other way to continue to maintain and grow it in a systematic and disciplined way. This issue is to discuss what is involved before we start looking into it.

[cortner]

• [x] rewrite the XScal1pBasis to be a generic 1PBasisComponent that stores a general basis; (cf PR #120 )
• [ ] explore whether there should be a separate continuous and discrete (categorical) B1pComponent
• [ ] implement frule for basis derivatives and rrule for model derivatives
• [ ] Implement a generic Compose object that will simplify the composition of basis pieces.
• [ ] Implement the linear transformation of the 1PBasisComponent as a transformation object of the innermost basis.

Rejected change (for now)

• [] remove the getval from the XScal1pBasis - cf. #124
• [ ] remove the transform from the polynomial basis and into getval - cf. #124