BornAgain usage examples.

Content:
- cpp        : How to use BornAgain from C++ (all other examples are in Python)
- scatter2d  : Scattering simulations with 2d detector
- specular   : Specular reflectometry examples
- varia      :
- fit...

All _simulation_ examples are under continuous test coverage
through Tests/Functional/Python/PyExamples/CMakeLists.txt.

Additionally, many _simulation_ examples are also tested
through Tests/Functional/Python/PyPersistence/CMakeLists.txt.

There is no autoamatic test coverage for _fit_ examples.
