API unification

[tpapp]

• refactor transformations interface, use informative keywords
• separate family and basis
• "bare" bases on [-1,1] no longer exposed by the API
• introduce univariate_basis
• refactor smolyak using levels
• re-enable JET, but with @report_opt or a function that exercises everything
• re-enable Aqua
• rewrite whole documentation
• use length instead of dimension?
• document that bases can be modified eg with Accessors.jl, properties are expored.
• document that when eg b = SmolyakBasis(Chebyshev(), Interior(), (nothing, nothing), SmolyakLevel(total = 2, each = 3)) (notice: no transformation), dimension(b) and length(grid(b))` still work, which is handy for calculations