Improve parallelism of ir_dist

[grst]

Closes #468

• [x] CHANGELOG.md updated
• [x] Tests added (For bug fixes or new features)
• [ ] Tutorial updated (if necessary)
• [x] test and compare performance with old implementation and full wu2020 dataset.

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (bd168a9) 80.52% compared to head (5255317) 80.50%.

Files	Patch %	Lines
src/scirpy/util/__init__.py	62.50%	3 Missing :warning:
src/scirpy/ir_dist/metrics.py	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #473 +/- ## ========================================== - Coverage 80.52% 80.50% -0.02% ========================================== Files 49 49 Lines 3994 4012 +18 ========================================== + Hits 3216 3230 +14 - Misses 778 782 +4 ```

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[grst]

In general this works well (also out-of-machine computing with dask), but:

• the tqdm progressbar is not compatible with the dask backend, or in fact pretty much any backend except for loky
  • instead, one can set the verbosity to ~5 to get some updates printed to the console. To make this nicer, maybe I can check if the backend is supported.
• for some reason it takes ~15 seconds to spawn the loky worker threads. When rerunning the function, this is instant. Not sure what the threads are doing initially.

[grst]

for some reason it takes ~15 seconds to spawn the loky worker threads. When rerunning the function, this is instant. Not sure what the threads are doing initially.

the reason was that the worker threads all import scirpy from scratch. It turned out to be quite slow because of import airr. I now deferred importing the airr package until it's needed (only in some IO functions) and this brought down the import time to ~1s which is acceptable and mostly due to scanpy.

[grst]

@felixpetschko fyi

In this PR I change how the ParallelDistanceCalculator handles parallel processing. It now uses joblib.Parallel that (amongst others) allows to use dask as a backend for multi-machine computing.

But I think your reimplementation of the hamming distance calculator anyway doesn't need this.