Merge branch 'feature/separate-extended-richards' into 'master'

Separate extended 1.5phase Richards model into a separate model

See merge request !3171

(cherry picked from commit 079d1f5e)

b2e36c96 Copy richards folder to richardsextended
e398bd2f [richards] Deprecate old extended Richards model
481da5ad [richards] Make ExtendedRichards inherit from base model
59f60b3c [richards] Make completeFluidState private
3f34a3ec [test][richards] Adapt test to new ExtendedRichards model