- adds missing header to files
- unify header
- add SPDX (https://spdx.dev/) identifier to header (first step for getting REUSE https://reuse.software/ compliant)
- reworked doxgen

- uses awesome doxygen theme: https://jothepro.github.io/doxygen-awesome-css/

Adresses: #183

- adds wiki pages to doxygen page
- adds doxygen modules for production code and tests

move velocity boundary conditions to BoundayConditions folder and deleted the following velocity boundary conditions:

BcVel27;
QVelDeviceCouette27;
QVelDevice1h27;
QVelDevCompPlusSlip27;
QVelDevCompThinWalls27;
QVelDevIncompHighNu27;
QVelDevCompHighNu27;
QVeloDevEQ27