Bugfix: Initialize communicator before grid generation.

Closes #28

See merge request irmb/VirtualFluids_dev!111

Moved Simulation::init() to Constructor, Simulation:free() to destructor.

See merge request irmb/VirtualFluids_dev!110

Moved Simulation::init() to Constructor, Simulation:free() to destructor and exchanged some members of Simulation with unique_ptr instances.

Using ENV for DEBIAN_FRONTEND is deprecated: https://github.com/moby/moby/issues/4032

Refactore the way Parameter is passed to EdgeNodeFinder

See merge request irmb/VirtualFluids_dev!108

- Parameter is passed as reference now (according to https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#f7-for-general-use-take-t-or-t-arguments-rather-than-smart-pointers)
- Where possible LBMSimulationParameter is passed instead of Parameter

Switch image version 1.3 and add VF_MPI compiler definition again for gpu version as well.

See merge request irmb/VirtualFluids_dev!107

Refactor routine for finding the edge nodes

See merge request irmb/VirtualFluids_dev!106

return std::optional from findIndexInSendNodes

Fix formatting in doxygen documentation

See merge request irmb/VirtualFluids_dev!105