Add Spline to m. module for user convenience

[wwells]

From PFA.general Slack:

srki [6:11 AM] Yes, it is clear for the polynomial regression. But if we need for instance to feet a smoothing spline function (smooth.spline {stats} from R) which is not straightforward to compute, we would practically need to translate the code of smooth.spline() into PFA expression using available PFA functions, right?

pivarski [1:21 PM] Yes. I didn't think of "canning" spline functions as named PFA callables, but that's not a bad idea. If I remember right, spline handle positions are something like a factor of three bigger or smaller than a tangent line to the curve at the control point. It's derivable, but maybe not fun to derive. That would be a good candidate addition to the "m." (math) module.