Add a new `Pyramid` class for dealing with pyramids more robustly

[pkgw]

Here we add a new toasty.pyramid.Pyramid class for better managing operations on tile pyramids. I've worked out some formalisms and ways to talk about the pyramid that I think will simplify a lot of things going forward. In this particular PR, we convert the "merge" (image cascade) functionality to use the new approach.

There is new functionality here in that we can now process "subpyramids", where we can do a reduction cascade over only a piece of a larger pyramid. This was necessary for DECaPS2 processing, where the full level=14 TOAST pyramid was going to take too long to process within a single process, even with a lot of CPUs. This is a slight generalization of our existing "tile filter" approach, but the complexity of implementing it within the earlier framework motivated me work out this new system.

One of the key improvements is the "reduction iterator" system for thinking about a depth-first pyramid walk as a reduction operation. The key thing is that it's not easy to parallelize reductions, so we can (should) only use the new iterator for relatively cheap computations. Image cascading must be done with the "walk" framework, which offers the possibility of parallelization but cannot "reduce". The basic problem with parallelizing reductions is transmitting data between processes, which is definitely solvable, but not something I want to deal with right now.

We use the new reduction iterator to speed up the initialization of parallelized, filtered TOAST cascades, because the previous implementation was basically walking the tree several times to compute all of the stuff that it needed to have to manage the parallelized cascade.

A nice extension would be to add a visit_leaves() method and port a bunch of our other operations over to use it. Should be pretty easy to do that now that the framework is in place.

[codecov[bot]]

Codecov Report

Merging #89 (f58ef47) into master (cd662ec) will increase coverage by 0.82%. The diff coverage is 93.69%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   75.61%   76.43%   +0.82%     
==========================================
  Files          24       24              
  Lines        3809     3981     +172     
==========================================
+ Hits         2880     3043     +163     
- Misses        929      938       +9     

Impacted Files	Coverage Δ
toasty/pyramid.py	93.07% <92.47%> (-1.58%)	:arrow_down:
toasty/merge.py	100.00% <100.00%> (+7.50%)	:arrow_up:
toasty/samplers.py	87.57% <100.00%> (ø)
toasty/toast.py	93.63% <100.00%> (+0.02%)	:arrow_up:

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[pkgw]

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