Merge branch 'fix/include-mess-in-solvers-hh' into 'master'

Fix the #include mess in solvers.hh

See merge request !137 (merged)

(cherry picked from commit ba9e6e7f)

5b2664ca [solvers.hh] Clean up the #include-mess. 42acfa9a [solvers.hh] Remove unused #include directives.