inputs/BRIT/specimen_flat/postprocess.sql: diameterBreastHeight_cm, height_m: updated runtimes

inputs/BRIT/specimen_flat/: DBH_*, Height_*: mapped NULL-equivalent values, using the steps at wiki.vegpath.org/Adding_new-style_import_to_a_datasource#Translating-filters-to-postprocessing-derived-columns

inputs/.../: translated multi-column filters with _avg() to postprocessing derived columns, using the steps at wiki.vegpath.org/Adding_new-style_import_to_a_datasource#Translating-filters-to-postprocessing-derived-columns

inputs/BRIT/specimen_flat/: translated single-column filters to postprocessing derived columns, using the steps at wiki.vegpath.org/Switching_to_new-style_import#stage-I-source-specific > "translate single-column filters to postprocessing derived columns"

/README.TXT: Maintenance: added instructions for what to do if http://vegbiendev.nceas.ucsb.edu/phppgadmin/ goes down (sometimes displaying a Not found error)

schemas/util.sql: schema comment: added note that IMMUTABLE SQL-language functions should never be declared STRICT, because this prevents them from being inlined. inlining can create a significant speed improvement (7x+), by avoiding function calls and enabling additional constant folding.

inputs/REMIB/Specimen/postprocess.sql: map_nulls() derived cols: documented total runtime (7.5 min on vegbiendev)

inputs/REMIB/Specimen/postprocess.sql: map_nulls() derived cols: updated runtimes for map_nulls() inlining, which created a speed improvement of 7x for the numeric columns and 2.5x for the text columns (292563.362->41929.772 ms and 83640.424->35690.797 ms, respectively). note that the map_nulls__coord__*() calls could be optimized further by combining the successive map_nulls() calls into one, with the hstores merged.

schemas/util.sql: map_nulls(): documented that inputs/REMIB/Specimen/postprocess.sql > country also shows that inlining is now happening properly. note that the speed improvement due to inlining is not as much, %wise, when the values util._map() is run on are long strings instead of the short strings used in the initial profiling. this is because a greater % of the time is spent in system functions such as hstore>text, which are not affected by the inlining because they are run either way.

schemas/util.sql: map_nulls(): use new nulls_map(). proper inlining (i.e. same runtime before and after change) has been verified with the following profiling query:
SELECT util.map_nulls(array[1, 2, 3]::text[], v) FROM unnest(array_fill(1, array¹⁰⁰⁰⁰⁰)) f (v)