add s1 indices

[felixlobert]

• adding the S1 Radar Vegetation Index (RVI) and the S1 backscatter cross-ratio to the list of available spectral indices

[davidfrantz]

better put in *.h file

[davidfrantz]

Hi Felix,

thanks for the contribution. However, the checks were not successful. There seem to be some syntax errors.
Please also take a look at the comment with the math include.

In addition, please be aware that there is a working branch under development, which may cause merge conflicts at a later stage: https://github.com/davidfrantz/force/tree/external_tables
However, as I currently don't have time to work on this branch, this should not be a reason to not accept your PR at this time.
Nevertheless, the routines for computing indices will change in the future as there will be an external table that defines wavelengths for each index.

Cheers,
David