Stem columns may be indicated by "dbh" as well as by "stem" in Gentry

Column headers vary from file to file in Gentry. We thought that
all of the stem column headers included the word "stem", but in
BEZA1 and BEZA3 (at least at the moment) they are indicated by
"SDBH". Since "dbh" doesn't occur elsewhere in the column headers
we now check this as well.
Explicitly check that all cells are empty for skipping lines in Gentry

We were originally checking that each row had at least 4 values and
that the first value wasn't empty, but in concept there could be
a missing value in the first column and in practice this is the
case for ELCORAZO.
Download missing gentry datafile separately

The main Gentry zip file that is supposed to contain all of the datasets is
missing CURUYUQU. This change checks to see if it is missing (in case of a
future fix from the data source), downloads the file separately, and adds it
to the list of files to be processed.
Manually fix incorrect column name in QUIAPACA gentry site

QUIAPACA has a column of counts, but it is mislabelled with the same header as the
DBH columns, STEMDBH. We have attempted to get the data provider to fix this and they
have not done so. This fix manually identifies the column correctly.

This change is fragile to an eventual fix by the data provider, but evidence over
the last several years suggests that such a fix is unlikely to occur.
Merge pull request #197 from ethanwhite/gentry-fixes

Fix issues with Gentry files