Enable running VirtualFluidsGPU without config file

Closes #22

See merge request irmb/VirtualFluids_dev!115

Small fix: Remove unused variable in QDeviceComp27

See merge request irmb/VirtualFluids_dev!117

Remove duplicate variables in boundary condition kernels and clean up some boundary condition kernels

See merge request irmb/VirtualFluids_dev!116

bug introduced in 07270713

The test needed to be changed because of commit #06fed8a5.

some fuctions were called twice

set defaults for gridPath and (out)Path
related #27 and #22

Clean up QVelDeviceComp27

See merge request irmb/VirtualFluids_dev!114

Use kernel managers to call the kernels (LBM, AdvectionDiffusion, BoundaryConditions)

See merge request irmb/VirtualFluids_dev!113