schemas/util.sql: added fold_explain_msg()

bugfix: schemas/util.sql: set_col_types(): need to COALESCE the executed SQL to '' because util.eval() does not support NULL (and shouldn't, because this indicates a missing COALESCE in constructing the statement)

schemas/util.sql: set_col_types(): use simpler util.eval() instead of manual EXECUTE/util.debug_print_sql()

schemas/util.sql: set_col_types(): use string_agg() instead of array_to_string(ARRAY) for clarity

schemas/util.sql: added mk_not_null()

added schemas/VegCore/Brad_Boyle/bien3_data_provenance_use_cases.docx* from e-mail from Brad

schemas/vegbien.sql: _plots_08_list_of_plots_which_use_percent_cover, _plots_15_pct_cover_of_each_verb_taxon_in_each_plot_in_each_pro: reran with fixes, which removes the incorrectly auto-added copies columns. (they were only able to be auto-added because the tables had no rows.)

bugfix: drop_column(regclass[]): need to run `SELECT NULL::void;` at end of function to avoid folding away functions called in previous query

fix: schemas/util.sql: diff(regclass, regclass): moved try_create() of copies column in parent table to auto_rm_freq() so that it would only happen if both tables actually contain a copies column (otherwise, the try_create() will create an empty copies column if both tables are empty)

schemas/util.sql: try_create(): also handle "child table is missing column" errors