GeneratorSpectators: add proton generation, divergence and crossing-angle sampling + fixes

[fcatalan92]

• Add GeneratorProtons/GeneratorSingleProton macros to generate spectator protons, mirroring the existing neutron generators.
• Add possibility to sample the beam divergence and beam-crossing angle from a uniform distribution each event, instead of a single fixed value (fixed-value setters still supported).
• Expose configurable parameters in macro functions.
• Fix BeamCrossing() calculation.
• Minor other fixes.

@coppedis @Fuligno