This function collects specified columns from L0_flat, converts into long (attribute-value) form by gathering variable_name. Regular expression matching joins unit to any associated variable_name and is listed in the resulting table's "unit" column.
"flat" format refers to the fully joined source L0 dataset in "wide" form with the exception of the core observation variables, which are in "long" form (i.e. using the variable_name, value, unit columns of the observation table). This "flat" format is the "widest" an L1 ecocomDP dataset can be consistently spread due to the frequent occurrence of L0 source datasets with > 1 core observation variable.