enforce T,S required for CTD, T for XBT, U,V for LADCP
implement latitude in thermal_wind and generalize automatically adding fields
fix thermal wind sign
add density and depth methods to CTDCast
add universal physical constants as global
add collection __eq__ and preserve cast type when reading
add collection reference data
change ctd_collection to castcollection and make type checking aware of cast subclasses
correct depth calc and use for thermal_wind
make section plots using primarykey rather than hard-coding pressure
add dateutil to requirements
add encoding header
fill preceding nans in rho when calculating depth
fix travis config