refactor xb.get_keepbits with xarray

[aaronspring]

I thought this PR would be more readable and faster because of more xarray operations and less looping, but I was so wrong on speed and also readability didnt improve that much:

ds = xr.tutorial.load_dataset("air_temperature")
info_per_bit = xb.get_bitinformation(ds, dim="lon", masked_value=None)

The only thing it does is saving us a few lines of code... @observingClouds do you think it is useful. I'm ready to close this PR without merge also.

New:

• get_keepbits now works on multi-dim inputs
• tests - how could we forget them before???

[observingClouds]

I currently get an error when trying

import xbitinfo as xb
import xarray as xr
ds = xr.tutorial.load_dataset("eraint_uvz")
info_per_bit = xb.get_bitinformation(ds, dim="longitude")
keepbits = xb.get_keepbits(info_per_bit, [0.999, 1.])

Seems like inflevel=1 is only allowed for single entries in this implementation.

[observingClouds]

I think refactoring this function would be great. I probably would prefer to split this function into smaller functions to have a better overview.

[observingClouds]

This looks great now! Thanks for also adding the tests! LGTM.