Move general functions to kval

[oyvfoss]

Where possible, want call general functions (which could be useful outside kobbe) in the kval library.

Should work on transferring useful general functions to kval.

List of potentially useful functions to be transferred:

• calc.mat_to_py_time()
• calc.runningstat(), calc.daily_average() <- Great, but should maybe use xarray functionality?
• calc.clean_nanmedian()
• load.chop() for deployment chop. Could be useful in a kval.data.mooring_tools module?
  • Nah, this one is pretty Sig specific.
• load.to_nc() - Probably belongs in kval.util.xr_funcs? (which maybe should just be kval.util.xr?)
• append.add_to_signdata (quite useful and might warrant a place in kval?).
• toolbox.plot_ellipse_icevel() - More general version would be great in kval..
• toolbox.histogram() - More general version would be great in kval..
• toolbox._uv_angle(u, v) - Definitely belongs in kval.ocean.

[oyvfoss]

toolbox._uv_angle(u, v) now has a counterpart in kval.ocean.principal_angle (haven't removed anything from kobbe yet, though)