Reach successfull XSuite <-> FLUKA exchange

[ghugo83]

This PR includes the following fixes:

• fluka_init_max_uid should also be called from XSuite, like in SixTrack. Otherwise, the server complains (from fluka_coupling/fluka/source_pre.f), and breaks the connection.
• pyfluka_set_synch_part should also be called from XSuite, like in SixTrack. Otherwise, the server complains (from fluka_coupling/fluka/source_pre.f), and breaks the connection.
• The reference particle info should be set, like in SixTrack. For example, it is now passed from xpart. Otherwise, the magnetic rigidity is accidentally null, hence all particles are cut: 0 particle received.
• Necessary to set a timeout value (at least on my local install). Otherwise, was getting a timeout error in FLUKA output file (lhc_run3_30cm001.out​).
• The I/O buffers from Python / C / Fortran were not in sync. Hence, printouts were spicing the debugging up: they were not in the order of actual runtime. I added a few flushes, this is definitely needed for debugging (for production, can just further be wrapped into a debugging mode).
• Shift of 1 in particle indexing, for partID and parentID. Lead to crash in FLUKA runs, from mgdraw.f.
• Shift of 1 in turn index.

Less important:

• Had a crash with python version check.
• Used npart instead of n_alloc as a parameter to pyfluka_init_max_uid (like in SixTrack). Under the hood, pyfluka_init_max_uid only sets a particle id: the min id for new particles to start from (no memory allocation). While looking at the received particles, this avoids having the new particles indices starting from e.g. 50000 (default n_alloc): instead, they simply start from npart + 1.
• Instead, kept n_alloc (instead of npart) for fluka_mod_init. Here, the situation is different, as under the hood, there is memory allocation (I presume this was done to avoid having to recompile, after changing the number of particles in example.py).
• Also set the e0​ and eof​ etc (mod_common), like is done in SixTrack (otherwise, they stay at their default value 0).
• Tried to keep the implementation as close as possible to the SixTrack one for now, to be able to easily compare printouts / results.

To be investigated for mental sanity:

• Check whether the collimator index has a shift by 1 or not.
• Not fully confident on the floating point precision used here and there.
• Quickly oversaw that the spin passed from SixTrack to FLUKA for protons is 0 (at least at the level of the connection, hence the info eventually received by FLUKA).
• The value for proton mass0 in XSuite is slightly different than the one in SixTrack (at least in fluka.log). SixTrack seems to eventually use a third value (taken from initial.dat).

Perspective:

• Run exactly the same particles distribution as SixTrack, and compare the lhc_run3_30cm001.out. Look at the results.
• Extend to multi-turn.
• Ideally, one could even try to get identical results. But not fully trivial: precision discrepancies from the different languages + have to set the FLUKA starting random engine state.

Should make @freddieknets happy.
Still a lot of further debugging to be done down the line.