Updated ExtensionProperties to include SamplingFeatures and Actions.

This update parallels the update to Annotations, to explicitly
demonstrate these cross reference tables that allow property/field
extensions to these tables.
Added SamplingFeatureNotes field

to SamplingFeatures table.
Modifying Little Bear River data use case script
Added Variables entity back to ODM2Core schema

The Variables entity went missing so I added it back.
Adding missing comments and relationships from the Variables table that went missing.  Modifying LBR script.
Added Variables Foreign Key back to ExternalIdentifiers schema
Update odm2_0.81_pgsql_emilio_changes.sql

Small update to comments
Changed Identifier URN to URI, again.

I missed this in 2 of five tables.
merging