Local Kriging module

[MuellerSeb]

As a first step towards a common base for GSTools and PyKrige, we should implement "local" Kriging.

That means:

• for each target point the kriging system is only solved in a specified neighborhood (either search-radius or n-nearest neighbors)
• reference could be: https://git.ufz.de/chs/progs/edk_nc/-/blob/develop/src/EDK_driver.f90
• these routines should be compatible with the functionality of GSTools' Krige class

Some prerequisites are already described in #9.
The netcdf interface is not necessary for now and could be added later on.

We need a plan to start working on this. Afterwards, we could start using this backend in a prototype of the upcomming PyKrige v2.