feat: add optional arg for id column name

[sarakolding]

• [x] I have battle-tested on Overtaci (RMAPPS1279)
• [x] At least one of the commits is prefixed with either "fix:" or "feat:"

Our ID is not usually dw_ek_borger (it's dw_ek_borger+admission_start_time), but when making the splits, we want to make sure that we are splitting by patient id too. Made some quick adjustments for it, but it can probably be done much simpler, and maybe it is not something we want on the main branch?

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Tests	Skipped	Failures	Errors	Time
47	1 :zzz:	0 :x:	0 :fire:	1m 4s :stopwatch:

[MartinBernstorff]

I think I need to understand why you're doing this to be able to review this PR :-)

Can you elaborate on why you would want to do it?

[sarakolding]

Since we kind of have two "layers" of groupings (admissions within patients, and days within admissions), we usually want to group at the highest level of granularity (individual admissions for individual patients). However, when we split our data for training, we want to make sure that individual patients are constrained within the same dataset, so one patient does not have admissions in both train and val. So our id is usually patient_id+admission_start, but specifically for splitting, we want it to be only patient_id. Hopes that makes more sense 😸