first_year and last_year in stream definition should be more general

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 https://github.com/ESCOMP/CDEPS/pull/61/commits/48a26e6e39a0a1dbfca506192fab089344f49725

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.