Feature/cleanup of eigenvalue solver docs

improve structure of the doxygen documentation to make the eigenvalue solvers visible and hide implementation details

enabled an automatic merge when the build for ecef33f0 succeeds

added 5 commits

• ecef33f0...76e26772 - 2 commits from branch master
• 0b84bf8e - [doc] explicitly structure the doxygen groups
• 2cd4ee71 - [doc] improve documentation of eigenvalue solvers
• 35a014f2 - move the solverhelper to solver.hh

Compare with previous version

added 3 commits

• ecd3e639 - [doc] explicitly structure the doxygen groups
• 040d59f5 - [doc] improve documentation of eigenvalue solvers
• 48a5fd86 - move the solverhelper to solver.hh

Compare with previous version

enabled an automatic merge when the build for 48a5fd86 succeeds

mentioned in commit b19bfdf0

merged