Specify three quantiles as required by corner 2.2.2

[johnveitch]

#297 finds a problem with corner v2.2.2 where it expects three quantiles to be specified. This updates the nessai default quantiles to include the 50% one (the corner default).

Closes #297

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (8e75c84) 99.49% compared to head (9853fd6) 99.48%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
- Coverage   99.49%   99.48%   -0.02%     
==========================================
  Files          61       58       -3     
  Lines        4593     4497      -96     
==========================================
- Hits         4570     4474      -96     
  Misses         23       23              

Flag	Coverage Δ
unittests	99.48% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nessai/plot.py	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

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[mj-will]

LGTM, thanks @johnveitch for addressing this. I'll try to make a bugfix release in the next couple of days with this change.