Fix MT random seed and SPLIT KISS random state restore

[mads-bertelsen-agentic]

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While looking around in the codebase I think I stumbled over a couple of problems with the random number generation that I here propose a fix for. This was discovered while using AI to analyze the current systems. Let me know if I misunderstood something and any of these fixes are actually necessary!

1: Mismatch in defines with MT causing seed to be ignored

mccode/src/cogen.c.in:2422-2424

#define _RNG_ALG_MT   1
#define _RNG_ALG_KISS 2

MT initialization is performed only by:
mccode-r.c:4259-4270

void mt_srandom(uint32_t seed) {
    mt[0] = seed;
    ...
}

mccode_main.c

#if RNG_ALG == 2
  mt_srandom(mcseed);
#endif

The above RNG_ALG sets the MT seed only when KISS is used instead of MT. Simple fix of RNG_ALG == 1 instead.

2: When the KISS random number generator is used (default), SPLIT needs to copy and restore the random state of each ray to continue the random number generation instead of redoing the same RNG. The KISS random number generate has 7 values on the particle struct, but only the first is copied / restored, so the quality of random numbers in SPLIT is reduced, easy fix to loop over the included values.

Questions in draft stage:

• Does this change in cogent.c.in constitute a change in the code generator that need a design decision entry?
• Test of MT, I did the run below, is that the correct approach?

MCSTAS_CFLAGS_OVERRIDE="-DRNG_ALG=1" \
mctest --instr=Unittest_SPLIT --testdir=run_split_mt --ncount 1E7 --limit 35 --skipnontest --mpi=3 

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