Get column names for the observational data frame
Value
Character vector of column names for observational data. The required
columns are Date (a UTC POSIXct; daily observations anchored at
12:00:00), var_aeme, depth and value. The optional columns are
depth_to (bottom of an integrated sample), sd (measurement standard
deviation, in the variable's units), depth_ref ("surface" or
"bottom": how the depth was recorded) and height_above_bed (metres above
the lake bed, for observations recorded relative to the bottom; depth is
always metres below the surface).
