Improved interface

[TheBB]

When this PR is done, the following should be supported and tested for B-Spline bases. Optimization is not a concern here.

• [x] Length: length(basis) => 5
• [x] Size: size(basis) => (5,)
• [x] Indexing: basis[i] => BSpline(...)
• [x] Domain: domain(basis) => (lft, rgt)
• [x] Evaluation:
  • [x] Single point, no derivatives, no coefficients: basis(x) => (vals, idxs)
  • [x] Several points, no derivatives, no coefficients: basis(xs) => [(vals, idxs)]
  • [x] Single point, derivatives, no coefficients: deriv(basis[, ord])(x) => (vals, idxs)
  • [x] Several points, derivatives, no coefficients: deriv(basis[, ord])(xs) => [(vals, idxs)]
  • [x] Single point, no derivatives, coefficients: basis(x, cs) => val
  • [x] Several points, no derivatives, coefficients: basis(xs, cs) => [val]
  • [x] Single point, derivatives, coefficients: deriv(basis[, ord])(x, cs) => val
  • [x] Several points, derivatives, coefficients: deriv(basis[, ord])(xs, cs) => [val]

And the following for basis functions.

• [x] Domain: domain(b) => (lft, rgt)
• [x] Evaluation:
  • [x] Single point, no derivatives: b(x) => (vals, idxs)
  • [x] Several points, no derivatives: b(xs) => [(vals, idxs)]
  • [x] Single point, derivatives: deriv(b[, ord])(x) => (vals, idxs)
  • [x] Several points, derivatives: deriv(b[, ord])(xs) => [(vals, idxs)]

Auxiliary tools:

• [x] Interval type for domains.