change the NaNs are handled in section plots

assume horizontal correlation. The output has no NaNs for right now, so
attention should be given to bathymetry
re-overlay NaNs after interpolating the filled array
make path handling more robust
move tests to subdir to avoid import name clashes
install pyproj from pip
add thermal wind calculation, bump version
purify functions
generated functions now have basic docstrings
calculate mask from cast to cast
implement mask switch
bump version
diff2 function to calculate gradients with NaNs
tweak readme
finish implementing thermal wind

when `thermal_wind` method is called on a `CastCollection`, it adds a dudz and a
U field to each cast
remove unicode names for python 2 compat :(
add scipy to runtime environment
force travis to use site-packages
travis: set system_site_packages to true
fix typo
not sure what I did last time, but this fixes the typo properly