Expose number of files option for logging SHM

[mileslucas]

Long story short I'm looking for the option to log a SHM and have it automatically stop logging after N cubes. Lo and behold, the logic seems to already exist in the c-code, but the variable is not exposed in the function, and is not propagated into any of the downstream code that relies on this (e.g., milk-logshim, any of the camstack code, etc.).

https://github.com/milk-org/milk/blob/3457402c102fdaced7026e6e89108f55c2e4fe88/src/COREMOD_memory/logshmim.c#LL662C36-L662C36

I presume this would just work, but has it been tried before? This kind of option would be very good to use when designing the control software for the VAMPIRES upgrade, and I think it would improve certain aspects of the FastPDI workflow.

I think a good CLI would be

milk-logshim <shm> <nsub> [<ncubes>] <directory>

such that

milk-logshim apapane 1000 /media/data

would do what you expect and is backward-compatible while

milk-logshim apapane 1000 10 /media/data

would stop after 10 cubes.