chore: Switch source calc from R version to python

This relates to developments to ease the maintenance
of the forecast system. With this development, there is no R code to
actively maintain. Moving from git project wheat-source-generation to
ews-source-generation. The python code has been validated for
identical performance to the R version. The only differences are
small changes from the underlying shapefile reading packages
(see notes-on-rewriting-in-python.txt in ews-source-generation).

Keeping the option to used the R code for some time in case any
follow-up investigation is required.