Use beam variants in the experiment class

[jbeilstenedmands]

This replaces the initial use of beam model templating in the experiment class with an approach using variants.

The idea here is that the experiment should be a simple container for the experiment models (beam, detector, crystal) etc, where each of these is a variant of allowed model classes. So far this is just done for beam:

Beam = std::variant<MonochromaticBeam, PolychromaticBeam>;

This is preferable to using base-class pointers as the different physical models have a different interface (e.g wavelength vs wavelength_range, get_s0 only exists for MonochromaticBeam). For the other models we might have variants being static and scan-varying models, single vs multi-panel detector, etc.

Then we define some beam helper functions in beam_ops to give type safe access and encapsulate the variant handling logic and error handling, with usage:

using beam_ops
auto& beam = require_monochromatic(expt.beam());

While I was here I split experiment.hpp into experiment.hpp and experiment.cxx, also moving out some beam serialization logic and tidied up the tests/CMakeLists.txt.

[dimitrivlachos]

Looks good!