schemas/vegbien.sql: implemented _plots_07_list_of_plots_which_use_counts_of_indiv_per_species

schemas/vegbien.sql, inputs/SALVIAS/validations.sql: added _plots_06a_list_of_stems, for use in figuring out the diff in _plots_06_list_of_plots_with_stem_measurements

schemas/vegbien.sql: plot: removed explicit column lists added in the autorename of plot.location_id->plot_id

schemas/vegbien.sql: plot: renamed pkey to plot_id. note that the field is autorenamed in all validation views which use it.

schemas/vegbien.sql: locationevent: added autopopulated plot_id column which points to the outermost plot of the locationevent's location

bugfix: schemas/vegbien.sql: locationevent: added missing fkey on place_visit_id

bugfix: schemas/vegbien.sql: _plots_06_list_of_plots_with_stem_measurements: only include stemobservation records which have actual stem IDs, not merely stem-related measurements (DBH, etc.)

bugfix: schemas/vegbien.sql: _plots_06_list_of_plots_with_stem_measurements: LEFT JOIN to project instead of inner joining, to get Postgres to use the right query plan. this is the last change needed to make query #6 runnable.

bugfix: schemas/vegbien.sql: rematerialize_out_view(): run all queries with `SET enable_seqscan = off` to avoid slow query plans. this fixes _plots_06_list_of_plots_with_stem_measurements and significantly speeds up _plots_10_count_of_individuals_per_plot_in_each_project (and possibly others).

schemas/vegbien.sql: locationevent: documented `CREATE INDEX locationevent_place_visit_id` runtime (3 min)