Add IDs to binary systems in synthetic clusters

[Gabriel-p]

Each star should be assigned integer IDs starting with 1 at first (when all stars in the sequence are generated), 2 to differentiate those below the magnitude cut (instead of discarding them entirely), 3 for those outside the mass limits, 4 for binaries, etc.

These isn't trivial and could increase significantly the time used by the synthetic cluster generation function.

This also complicates matters with binarity assignation (see #198), since keeping stars beyond the max-mag-cut means most stars selected to be binaries will be located beyond the max-mag-cut limit simply because most stars are located in the lower regions of the sequence.

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The above is not necessary, provides no immediate benefit, and could disrupt the process of generating a synthetic cluster quite a bit.

Instead, add IDs to differentiate single stars from binaries.