Enhance (and rename) nicer_slicer -> longitude_slicer

[navidcy]

Closes #32

[navidcy]

I noticed

regional-mom6/regional_mom6/regional_mom6.py

Line 207 in 544eb43

## Here we assume that data has equally spaced longitude values spanning 360 degrees.

Let's add an assert to ensure our assumption is correct!

[navidcy]

I also noticed

regional-mom6/regional_mom6/regional_mom6.py

Line 161 in 544eb43

data wraps around (commonly either in domain [-180, 180] or in [-270, 90]).

Why isn't [0, 360] not part of the "commonly used domains"?

[navidcy]

I think nicer_slicer should be renamed to longitude_slicer, right?

[navidcy]

Question:

Here:

regional-mom6/regional_mom6/regional_mom6.py

Lines 165 to 176 in 544eb43

	- Determine whether we need to add or subtract 360 to get the
	middle of the ``xextent`` to lie within ``data``'s longitude range
	(hereby ``oldx``).
	- Shift the dataset so that its midpoint matches the midpoint of
	``xextent`` (up to a muptiple of 360). Now, the modified ``oldx``
	does not increase monotonically from West to East since the
	'seam' has moved.
	- Fix ``oldx`` to make it monotonically increasing again. This uses
	the information we have about the way the dataset was
	shifted/rolled.

we mention oldx but I don't see any oldx in the source code. Is the oldx only for reference in the docstring or was there an oldx variable at some point that got deprecated?

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[navidcy]

Ideally this PR should also deal with #118. I'm not sure if the demo notebook uses this slicer (and that's how my need for #119 came about).

[ashjbarnes]

Looks good. Might need to merge renaming to longitude_slicer will merge nicely with #122