Remove problematic use of catch_warnings

[stephenworsley]

📰 Custom Issue

Part of the investigation in #5499 found that warning duplication was being caused during loading by these lines of code:

iris/lib/iris/fileformats/_nc_load_rules/helpers.py

Lines 417 to 431 in b6e39d7

	with warnings.catch_warnings(record=True) as warning_records:
	cube.cell_methods = parse_cell_methods(nc_att_cell_methods)
	# Filter to get the warning we are interested in.
	warning_records = [
	record
	for record in warning_records
	if issubclass(record.category, UnknownCellMethodWarning)
	]
	if len(warning_records) > 0:
	# Output an enhanced warning message.
	warn_record = warning_records[0]
	name = "{}".format(cf_var.cf_name)
	msg = warn_record.message.args[0]
	msg = msg.replace("variable", "variable {!r}".format(name))
	warnings.warn(message=msg, category=UnknownCellMethodWarning)

This code should be refactored to not use catch_warnings and it should be demonstrated that this reduces warning duplication.

[stephenworsley]

Note: the success of this task will influence the decision made regarding the warning workarounds proposed in #5499. If enough of the warning duplication is solved by this ticket we may deem such workarounds to be no longer necessary.