add initial capability for specifying layer bounds refs #2941 #2567
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 pull request #3023 from friedmud/layer_bounds

add initial capability for specifying layer bounds