Add Splines of order 4 - 7

[rodrigoacb]

This commits add BSplines of order 4 to 7 following the polynomial definitions on deserno1998 [1]

Following @streeve request, PR #799 is now split into two PRs and once merged, this one will only show diff changes related to Splines.

This changes were done in cooperation with @rhalver

[1] Markus Deserno, Christian Holm; How to mesh up Ewald sums. I. A theoretical and numerical comparison of various particle mesh routines. J. Chem. Phys. 8 November 1998; 109 (18): 7678–7693. https://doi.org/10.1063/1.477414

[streeve]

@lebuller can you take a look at the spline updates here? @rodrigoacb probably easiest to rebase out all but the first commit

[rodrigoacb]

@lebuller can you take a look at the spline updates here? @rodrigoacb probably easiest to rebase out all but the first commit

Thanks @streeve Sam. I rebased onto the first commit and kept the clang-format commit since lt changed files related to this pr

[streeve]

ping @lebuller