add __ne__ methods

Conflicts:
	narwhal/cast.py
fixed, remove print statement
split into util and plotutil to avoid circular dependency
thermal_wind_inner fixes
use plotutil
finish thermal_wind_inner implementation
add incomplete tricontourf-based plotting
remove  implementation because it was wrong
add diff2_dinterp function
add kernelsize option to plot_section_properties
split IO tests into seperate file
add LADCP shear method
add_shear takes sigma as an optional argument for pre-smoothing
fix GSW abs sal mentioned by cr; add methods for searching a cast collection by property key
fix add_density test
dispatch to cast.add_density for thermal wind
add option to flip proj dist
plotting refinements and bug fixes
minor name fixes
refactor plt_section_properties functions by splitting out the interpolation code and allowing them to share common code for boring plot steps