feat: created transform/integrate.py and modeled after select.py

[msteckle]

I'm submitting a PR but you shouldn't merge it for now because I just want to chat about it first. I had some questions I want to write down:

• For these transforms, we're returning the ds if it doesn't meet some requirement rather than failing. I know you said you wanted it to be hard to fail, but now I would think the user is expecting to have transformed the data but they don't know that it actually didn't transform, is that right?
• For integration; caveat: I don't have really any calculus knowledge at all, so I was basically having to look up what integration was and why we should use it instead of just regular old summation. I've got these reasons:
  • Not regularly spaced data (so stretched lat/lon, irregular time steps, etc)
  • Maintaining the physical units
• Then I found that xarray has .integrate() and I was wondering why you did var.weighted(msr.fillna(0))
  • Then I got stumped on what the general .integrate() function does; do we want a generalized integrate function that still works if a user supplies some other mystical dimension besides lat/lon/time/depth? If so, users could just specify trapezoidal (xr.integrate) or weighted sum (what you used in the code)

Anyway, that's a lot. I'm basically not seeing why we need an integrate() base class when it just uses one of the child classes (integrate_time, integrate_space, integrate_depth) and doesn't actually do anything on it's own.

I also haven't tested this since I wanted to discuss first.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.00%. Comparing base (62bf337) to head (e377517).

Files with missing lines	Patch %	Lines
ilamb3/transform/integrate.py	0.00%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
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- Coverage   69.67%   69.00%   -0.67%     
==========================================
  Files          37       38       +1     
  Lines        3307     3339      +32     
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- Misses       1003     1035      +32     

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[nocollier]

I think you do not need this block at all.

[nocollier]

if self.dim == "time":   # we configured this integrate for time
  if not dset.is_temporal(ds): return ds  # but the ds isn't temporal
  ds[self.varname] = dset.integrate_time(ds, self.varname, mean=self.mean)

[nocollier]

ditto the others