Moves regression-tests to tests/

- 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

- fix error in the viscosity calculation
- remove confusing variable "length"
- add default config file that works in the docker container
- set initial velocity

move slip boundary conditions to BoundaryConditions folder and deleted the following boundary conditions:

QSlipDev27;
BBSlipDevComp27;
QSlipPressureDevCompTurbulentViscosity27;
QSlipGeomDevComp27;
QSlipNormDevComp27

https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/issues/148

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

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

Merge branch 'develop_temp' of https://git.rz.tu-bs.de/h.alihussein/VirtualFluids_dev into develop_temp