Material refactoring

See merge request !489

Merge BasicVector3D into Vectors3D.h, make it header only, and decouple from Complex.h

See merge request !486

Correct ctest commands which need setting environmental variables

See merge request !484

compute sinc(x) diretly w/o calling gsl

See merge request !485

The correct platform-independent CMake command to set environmental variables is
```
add_test(NAME <test-name> COMMAND ${CMAKE_COMMAND} -E env ...)
```

Otherwise, CMake assumes the existence of an executable like `env` (or `env.exe`
under Windows). Under Windows, this will lead to an error when such tests are
executed via `ctest`.

mv polyhedral base classes to LibFF

See merge request !483

Refactor SimulationOptionsItem (no SessionItem, no SessionModel any more)

See merge request !481

BasicVector3D now inheriting from std::array

See merge request !482