Add DELTA_MINUS_VAR to L2 map epoch variable

[greglucas]

Add a zero variable for epoch_delta_minus on the maps.

Jon:

The decision was to make the Epoch the start of the sample instead of middle (which is weird but, hey, maps are
weird). In that sense the DELTA_MINUS is not strictly necessary, because they're marked with the BIN_LOCATION as 0.0. But maybe it makes sense to update our map metadata to include a DELTA_MINUS_VAR that's just a
non-RV zero?

Jamey:

Sounds reasonable to me to add the delta_minus_var as equal to 0 in this case, so that all variables are consistent across CDFs.