Small optimisations for statically-sized bases

[jipolanco]

This PR allows defining statically-sized B-spline bases. To do this, the breakpoints used to define a basis must be stored in a tuple or an SVector:

julia> breaks = (0.0, 0.1, 0.2, 0.6, 1.0);

julia> B = BSplineBasis(BSplineOrder(3), breaks)
6-element BSplineBasis of order 3, domain [0.0, 1.0]
 knots: [0.0, 0.0, 0.0, 0.1, 0.2, 0.6, 1.0, 1.0, 1.0]

julia> knots(B)
9-element StaticArraysCore.SVector{9, Float64} with indices SOneTo(9):
 0.0
 0.0
 0.0
 0.1
 0.2
 0.6
 1.0
 1.0
 1.0