Spline generation is type unstable

[rafaqz]

We have multipile ntuple calls with length based on run-time values. This cant be type-stable by definition.

DataInterpolationsND.jl/src/spline_utils.jl

Lines 77 to 80 in d1baa7b

	basis_function_values = ntuple(
	k -> (k == degree_plus_1) ? one(T) : zero(T),
	degree_plus_2
	)

This instability is the majority cost of running spline interpolation. It can be fixed by moving degree to the type and making sure the +1 and +2 also compiles away.

[rafaqz]

Additionally types are passed passed into closures, which looks kind of fine but isn't as its a struct field underneath

DataInterpolationsND.jl/src/spline_utils.jl

Line 71 in d1baa7b

return ntuple(_ -> zero(T), degree_plus_1)

Will fix all of this with other performance improvments.