add support for SHTns

[jacksilver045]

SHTns[1] is a "blazingly fast" library for Spherical Harmonic Transform. It is used in gfs-dycore[2] instead of spherepack and Jeffrey Whitaker provided a nice shallow water example in python with some wrappers[3] to equivalent pyspharm functions. It should be straitforward to implement into windspharm as an optionnal backend.

[1] http://users.isterre.fr/nschaeff/SHTns/
[2] https://code.google.com/p/gfs-dycore/
https://code.google.com/p/gfs-dycore/source/browse/trunk/src/shtns.f90
[3] http://users.isterre.fr/nschaeff/SHTns/shallow_water_8py-example.html

[ajdawson]

I've been thinking about this myself recently. Thanks for providing the links, they'll be helpful.

I did investigate this a year or so ago but had no luck getting the SHTns code working correctly for some simple use cases, hopefully things will be different this time.