Adding beam options in hxsim

[chiaratomaiuolo]

In hxsim.py, the test beam is generated inside the simulation as a GaussianBeam. It would be useful to add an option for choosing between different types of beams, such as a UniformBeam (to be implemented, useful for the hardware simulation of the custom readout chip) or a PointBeam(this is less interesting, it can be simulated using a narrow GaussianBeam), useful for testing.

[chiaratomaiuolo]

The UniformBeam has been implemented roughly defining the square containing the whole readout hexagonal grid. Now it is the right thing to do because every event that we generate should trigger. This should be modified when issue #49 will be closed and the MC structure changed.