Don't allow start_yr, end_yr, and align_yr to be set in user_nl_datm_streams when it's set by the xml variables DATM_YR_*

[ekluzek]

Currently you can set start_yr,end_yr and align_yr to be set both by user_nl_datm_streams and by setting the DATM_YR_* xml variables with xmlchange. Since, this can lead to a contradiction it would be good to disallow it being set by user_nl_datm_streams and the user told to modify it with xmlchange.

We do this same sort of thing in other components as well such as CTSM.

[ekluzek]

We think the way this is done is to do the following kind of changes:

diff --git a/datm/cime_config/stream_definition_datm.xml b/datm/cime_config/stream_definition_datm.xml
index af79283..f4c6687 100644
--- a/datm/cime_config/stream_definition_datm.xml
+++ b/datm/cime_config/stream_definition_datm.xml
@@ -220,9 +220,9 @@
       <mapalgo>none</mapalgo>
     </stream_mapalgo>
     <stream_vectors>null</stream_vectors>
-    <stream_year_align>$DATM_YR_ALIGN</stream_year_align>
-    <stream_year_first>$DATM_YR_START</stream_year_first>
-    <stream_year_last>$DATM_YR_END</stream_year_last>
+    <stream_year_align modify_via_xml="Set this with the DATM_YR_ALIGN xml setting" >$DATM_YR_ALIGN</stream_year_align>
+    <stream_year_first modify_via_xml="Set this with the DATM_YR_START xml setting" >$DATM_YR_START</stream_year_first>
+    <stream_year_last  modify_via_xml="Set this with the DATM_YR_END xml setting" >$DATM_YR_END</stream_year_last>
     <stream_offset>0</stream_offset>
     <stream_tintalgo>
       <tintalgo>linear</tintalgo>