[doxygen] added Richards tests

doc/doxygen/modules.txt

...mediumflow/richards/implicit/richardsanalyticalproblem.hh

...flow/richards/implicit/richardsanalyticalspatialparams.hh

...porousmediumflow/richards/implicit/richardslensproblem.hh

...mediumflow/richards/implicit/richardslensspatialparams.hh

...diumflow/richards/implicit/richardsniconductionproblem.hh

...diumflow/richards/implicit/richardsniconvectionproblem.hh

...usmediumflow/richards/implicit/richardsnispatialparams.hh

...diumflow/richardsnc/implicit/richardswelltracerproblem.hh

...ow/richardsnc/implicit/richardswelltracerspatialparams.hh