Merge branch 'rename-elasticenergyfunctional-to-integralfunctional' into 'master'

Explicitly request visitor from the ::Impl namespace

See merge request !73
2 jobs for master in 11 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #89881
dune:git clang C++17

00:07:55

passed #89882
dune:git gcc-8 C++17

00:11:26