Add new features to meanify & make compatible with last version of scipy.

[PFLeget]

Add features to meanify to scale in O(1) in memory for mean stat and to compute in sky coordinates.

[codecov]

Codecov Report

❌ Patch coverage is 96.64430% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.13%. Comparing base (b705b7b) to head (83609db).

Files with missing lines	Patch %	Lines
treegp/meanify_healpix.py	96.00%	3 Missing ⚠️
treegp/meanify.py	96.96%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   87.42%   89.13%   +1.71%     
==========================================
  Files           8        9       +1     
  Lines         803      939     +136     
==========================================
+ Hits          702      837     +135     
- Misses        101      102       +1     

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