`DATETIME_DIFF` implementation varies between postgres and bigquery

[alistairewj]

As raised in #1529, DATETIME_DIFF in PostgreSQL calculates a fractional number, whereas DATETIME_DIFF in BigQuery returns a whole number. This results in a subtle divergence of the concepts in the two dialects. urine_output_rate may be affected as various comparisons expected fractional values to be returned:

mimic-code/mimic-iv/concepts/measurement/urine_output_rate.sql

Lines 44 to 53 in a830870

	-- note that we assume data charted at charttime corresponds
	-- to 1 hour of UO, therefore we use '5' and '11' to restrict the
	-- period, rather than 6/12 this assumption may overestimate UO rate
	-- when documentation is done less than hourly
	, SUM(CASE WHEN DATETIME_DIFF(io.charttime, iosum.charttime, HOUR) <= 5
	THEN iosum.urineoutput
	ELSE null END) AS urineoutput_6hr
	, SUM(CASE WHEN DATETIME_DIFF(io.charttime, iosum.charttime, HOUR) <= 5
	THEN iosum.tm_since_last_uo
	ELSE null END) / 60.0 AS uo_tm_6hr

Easiest fix is to (1) update postgres-functions.sql and (2) update the UO query to take a similar approach to the kdigo_uo query.