first_year and last_year in stream definition should be more general

[billsacks]

I noticed that many (all?) of the stream_definition xml files define a first_year and last_year like this:

https://github.com/billsacks/CDEPS/blob/48a26e6e39a0a1dbfca506192fab089344f49725/datm/cime_config/stream_definition_datm.xml#L206

I see two problems with this:

1. If you change the start & end years defined in config_component, you need to also change these years; it isn't obvious that you need to do this.
2. I think this reduces flexibility for a user who may want to set custom start & end years for forcing data for their case.

A simple fix is to expand these years to have a ridiculously huge range, as I did in 48a26e6

I think a cleaner fix would involve making first_year and last_year optional here. From a quick glance through the code that parses this, it looks like these may currently be required.

[billsacks]

I did some testing that suggests that first_year and last_year are currently required for the list of stream files to be created correctly. This seems to be the responsible code:

CDEPS/cime_config/stream_cdeps.py

Lines 101 to 111 in 9f54ef0

	if 'first_year' in child.xml_element.attrib and 'last_year' in child.xml_element.attrib:
	value = child.xml_element.get('first_year')
	value = self._resolve_values(case, value)
	stream_year_first= int(value)
	value = child.xml_element.get('last_year')
	value = self._resolve_values(case, value)
	stream_year_last = int(value)
	year_first = max(stream_year_first, data_year_first)
	year_last = min(stream_year_last, data_year_last)
	stream_datafiles = self._sub_paths(stream_datafiles, year_first, year_last)
	stream_datafiles = stream_datafiles.strip()

My initial thought on how to resolve this is:

• Have separate conditionals for first_year and last_year (that way you can have open-ended ranges, such as: one range that goes from any start year up through 1999 and another range that goes from 2000 through any end year)
• For each of these conditionals: if the attribute (e.g., 'first_year') is present, then use the current logic; but then add an else block for the attribute absent, where it simply sets year_first (or year_last) to data_year_first (or data_year_last)
• Then the last two lines (setting stream_datafiles) should be done outside of any conditional

I haven't tested this.

[ekluzek]

This is related somewhat to #82. @billsacks we should likely discussion this more...