Reorganize code among files in core

[visr]

Apologies for the huge diff and the conflicts this will create, but a reorganization of the core code was overdue. This only moves code, there are no other changes.

A rough overview of the changes:

• validation.jl: split out schemas.jl, and moved all validation functions in
• solve.jl: split out parameter structs to parameter.jl
• create.jl and io.jl: split out into read.jl and write.jl
• const.jl: move single const to write.jl
• add callback.jl for solver callback related code
• add graph.jl for graph related code
• bmi.jl: split out all non-BMI functions, moved large function bodies elsewhere
• utils.jl: split out add sparsity.jl for Jacobian sparsity generation

[Hofer-Julian]

It's not really easy to review this, but the mentioned changes look fine.
Also the tests are green.

[visr]

Ok I'll go ahead and merge this. I can help updating open PRs if needed.