[dune.module] do not depend on dune-fem and dune-fem-localfunctions anymore
[basefunctionset.fem] add proper guards
[autotools] make eigen optional
[mapper.fem] add proper guards and errors
[space.continuouslagrange.fem-localfunctions] add proper guards
[space.continuouslagrange.fem] add proper guards
[space.discontinuouslagrange.fem-localfunctions] add proper guards
[tests.elliptic-cg-discretization] calculate grid width by hand
begin transition from grid parts to grid views
[tests.elliptic-testcases] replace grid parts by views
[tests-elliptic-cg-discretization] use pdelab space and grid views