[FEATURE]: Rename spatial_avg.py to spatial.py

[tomvothecoder]

Is your feature request related to a problem?

While working on #107, I noticed that I was adding temporal functions/methods to temporal_avg.py that weren't directly related to the TemporalAverageAccessor class. For example, the TemporalAverageAccessor.center_times() should actually just be a standalone function that lives in the .py module.

I ended up renaming the file from temporal_avg.py to temporal.py to store all temporal related functions and classes.

If we want to group functionality based on axes/dimensions, we should also rename spatial_avg.py to spatial.py.

Describe the solution you'd like

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Describe alternatives you've considered

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Additional context

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[tomvothecoder]

Hey @pochedls, let me know what you think about this.

[pochedls]

Hey @pochedls, let me know what you think about this.

I like this a lot.

[durack1]

@tomvothecoder I think this is ideal, you may want median, min, max, ..., operations in addition to average so more general axes/spatial and time time_coordinate/temporal is a smart move. Just thinking aloud, I wonder if vertical would be folded into axes, or would be its own class?

@pochedls has the arbitrary index discussion come up anywhere, so what I mean is captured in CDAT/cdat#1288 - this very well maybe already a core part of xarray