Minus signs for recorded absences in Explorer #22

Ordinarily, absences for present/absent data are not recorded in the database, so they appear as blank cells in the spreadsheet. However, occasionally, the recording of absences is warranted, and these appear as 0 values in the Data table. In the Explorer spreadsheet, presence values are converted from 1 to a plus '+' sign. To be consistent, a 0 should be converted to minus '-' sign. An example dataset is DatasetID 26811 from Laguna Ek'Naab.