schemas/util.sql: added ifnull(), equivalent to MySQL's IFNULL (Postgres auto-lowercases the name)

schemas/vegbien.sql: public_validations_*: merged into single public_validations schema, with type-specific prefixes for each query, so that datasources can use validations queries from multiple type categories, and so that each datasource doesn't have to indicate which validations output schema it's using

schemas/util.sql: diff_cols(): documented how to run EXPLAIN on the FULL JOIN query

schemas/util.sql: diff_cols(): embed left_query/right_query directly in the main SQL statement, so that the query planner can take the sort order of the queries into account in planning the FULL JOIN (and where possible, use a fast merge join)

schemas/vegbien.sql: regenerated from DB, which adds an autogenerated column alias

schemas/vegbien.sql, validation/aggregating/*/*.sql: renamed public_validation_* to plural public_validations_* since we refer to them as aggregating validation*s*

schemas/util.sql: added eval2col_pair()

schemas/vegbien.sql, validation/aggregating/*/*.sql: renamed public_validation_* to plural public_validations_* since we refer to them as aggregating validation*s*

schemas/vegbien.sql: public_validation_plots: added Brad's notes as comments

schemas/vegbien.sql: public_validation_plots: populated queries from validation/aggregating/plots/bien3_validations_salvias_vegbien.sql