Gcvspline

- Add a minimal pyproject.toml file

- Fix of a documentation issue under Python 2.7.

- major change in API

- gcvspline and splderivative that are direct wrappers of the FORTRAN functions. See gcvspline_fortran_interface.py for an example.

- Scipy-like interface of several functions: GCVSmoothedNSpline, MSESmoothedNSpline, DOFSmoothedNSpline, SmoothedNSpline. See ./examples/gcvspline_scipylike_interface.ipynb for examples.

- gcvspline and splderivative are direct wrappers of the FORTRAN functions. See example.py for an example.

- Wrap of the GCVSPL.f code with f2py, broken with new Python versions.

- Registration of gcvspline in PyPi.