I have just merged the branch 'no-ig' (aka element groups, go away). The element groups served no particular purpose for some time already, because the fields required the same element type in the whole mesh anyway. As you can see in the merge message below, I was mostly deleting and simplifying code:
63 files changed, 1761 insertions(+), 3931 deletions(-)
The functionality stays, while there should be a lower memory footprint and the code is slightly faster (about 10% when running tests on my computers).
Your code will probably need an update, if you access material data or physical quadrature points directly. If you never needed to know about element groups, you probably do not need to do anything.
There might be also some left-overs in the code/docs, so let me know about any problems or inconsistencies.