Add gradient calculation for surfaces

[egiovan]

First. It is a very nice and useful project the modernization of the Dierckx routines, so, thanks.
A useful addition will be the calculation of the gradients for a gridded surface. The Dierckx parder function is already in fitpack_core, so it shouldn't be difficult. One has only to decide if one wants a different function or wants to add some optional arguments to the gridded_eval_ functions.
I may easily do it if you like.

[egiovan]

There could be a problem with the length of the work array as it may be bigger in parder. But I have seen that you have written pardtc that can return the derivative of a surface. That is very nice. One can add a method that return a derivative of a spline as another spline.

[perazz]

Thanks for the interest @egiovan. Do you have a simple sample case where this could be evaluated?
Currently, we only have a parder test from the original testsuite that consider a square cartesian grid

[perazz]

@egiovan I've implemented the derivative evaluation for both cartesian bivariate types:

fitpack/src/fitpack_surfaces.f90

Lines 83 to 88 in 68444ec

	!> Evaluate derivatives at given coordinates
	procedure, private :: surface_derivatives_gridded
	procedure, private :: surface_derivatives_many
	procedure, private :: surface_derivatives_one
	generic :: dfdx => surface_derivatives_one,surface_derivatives_many
	generic :: dfdx_ongrid => surface_derivatives_gridded

I don't have a quick test to run on them, so if you do, please let me know, happy to add a test case.