Fixes #2861 .  This does not change any C++ code.  Documentation is updated, mostly concerning mass lumping.  Some more tests utilising mass lumping are added
Merge pull request #3025 from WilkAndy/feature_2861

Fixes #2861 .  This does not change any C++ code.  Documentation is upda...
Merge commit '0dfd6afc7bf8afeee329d2cebb7987560d61ae26'