Improving pyroms installation: csa

[hadfieldnz]

Hi Kate

I am continuing with my programme of rationalising and/or streamlining and/or documenting the components that make up pyroms and I have come to csa (pyroms/external/csa). From the contents of this directory, this is Pavel Sakov's Bivariate Cubic Spline approximation library + standalone utility, Version 1.21. The make file installs a library (libcsa.a), a binary (csabathy), a header file (csa.h) and maybe some other stuff, but not a Python module. csa is involved in the build process for gridutils (pyroms/external/gridutils/makefile, etc) and is also mentioned in a comment in pyroms/external/nn/delaunay.c, but not actually used.

There is now a Conda csa package on conda-forge (https://github.com/conda-forge/csa-feedstock) referring to the home page at https://github.com/sakov/csa-c. The Conda package is at version 1.22.0. When installed, it gives the library, the binary and the header file.

I therefore propose to delete the csa subdirectory from the pyroms repository, but mention csa in the README file as an optional Conda package.