Support returning total bed days and total critical care days from patients.admitted_to_hospital

[rebkwok]

Fixes #742

patients.admitted_to_hospital already returns days_in_critical_care, however this will return the number of days for the first or last matching admission.

This allows the return options of total_critical_care_days_in_period and total_bed_days_in_period by summing the days for all admissions during the period. In the case where there are two or more admissions recorded with the same admission date, we take the max number of days to avoid counting the sames days more than once.

For total_bed_days_in_period, the return value required is number of days, but the data recorded is number of nights (sometimes 0, which counts as one bed day).

[iaindillingham]

This is really clear. Thanks for commenting on the adjustment, and making it explicit in the code.