api refactoring

[tpapp]

The package has seen enough use (mostly for projects that I am involved in, as far as I know) to warrant some refactoring and API revamp. This is a list of ideas I want to work through, some may be split into a later PR, let's see. This may result in releasing version 1.0.

API

• rename dimension to basis_dimension and domain to basis_domain, to avoid clashes with other packages. These terms are not generic anyway.
• a unified function make_basis(family, grid_kind, dimspec) should replace univariate_basis and Chebyshev. family should admit Smolyak(Chebyshev). dimspec should allow integers as a shortcut, but maybe convert to types.
• an API to tensor mixed setups, eg full tensored x smolyak

Internals

• bases should have their dimension available statically (as part of the type), cf make dimension part of the type #32
• calculations should unroll for at least low dimensions, especially Smolyak, benchmark this
• the iterator form of linear_combination may not be ideal for all work, implement one that reuses common subexpressions, unrolling
• optimize for linear_combination, most important
• second most important: fast grid calculation
• get a Tensor basis, cf this discussion. Implemented with the above features in mind. API could follow a "product" approach, eg overload ×

Open questions

• a simple API for fitting "constant" and "linear" models within a family.