Reduce default target_chunksize

[astrofrog]

I think 1e6 is probably a more sensible value in case cubes are rechunked to combine chunks along any of the dimensions.

[codecov-io]

Codecov Report

Merging #11 (366b6d1) into main (a077f37) will decrease coverage by 39.92%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             main      #11       +/-   ##
===========================================
- Coverage   93.72%   53.80%   -39.93%     
===========================================
  Files           4        4               
  Lines         526      526               
===========================================
- Hits          493      283      -210     
- Misses         33      243      +210     

Impacted Files	Coverage Δ
casa_formats_io/casa_dask.py	11.57% <0.00%> (-83.48%)	⬇️
casa_formats_io/casa_wcs.py	22.72% <0.00%> (-75.76%)	⬇️
casa_formats_io/casa_low_level_io.py	87.36% <0.00%> (-3.35%)	⬇️

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