add travis branch back to testing to fix tests
update reference tests to have a date field
fix for saving properties, and convert dates to datetime objects when reading
ensure that saved files have 'nwl' extension
import subclasses of Cast
recognize properties labelled 'time'
load support for non-standard primarykey and set default primarykey for LADCP to "z"
fix bottom key lookup and avoid modifying arguments
note that thermal_wind calc uses pressure rather than depth
add reference for test
make station lines dark grey
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