Allow customization of interpolation degree in EOP
EOP interpolation uses a fixed number of points for interpolation, and this number of points is the same for data that has no derivatives and uses Lagrange (like dx and dy) and for data that does have derivatives and uses Hermite (like xp, yp). Also UTA/LOD should use derivatives since LOD is basically the opposite of UT1 derivatives (scaled to one day). It would be better for users and more consistent between parameters to set up an interpolation degree, with some default. As some parameters use Hermite with first derivatives and for symmetry reason we want an even number of points, the degree should have the form 4k-1, typically 3 (4 points without derivatives, 2 points with derivatives) or 7 (8 points without derivatives, 4 points with derivatives).