Final updates to Step 1 code - read data, run ddf

This version now cleanly extracts BBS transect and square-level bird and
habitat data and runs a ddf model with visiblity + visit as covariates -
these are used for modelling CU detectability, but may not be appropriate
or other species, in which case one of the other models (e.g. that
includes habitat) may be better suited.