Angular correlation code adde to extra.utils

[TimBerberich]

This adds extra.utils.xcca which contains routines to compute cross-correlations of polar data with and without detector mask.
It also contains routines to compute cumulative/(iterative) averages and variances which are useful if you want compute the average ccf by splitting your entire dataset in parts and then merging computed partial results.

Example:

from extra.utils.xcca import AngularCorrelator, AveragedAngularCorrelationMasked
import numpy as np

data = np.random.rand(100,32,64)
mask = np.random.rand(100,32,64)>0.7

ac = AngularCorrelator(*data.shape[1:])

#compute cross-correlation
ccf  = ac.ccf(data[0],mask=mask[0])
#compute harmonic coefficients of cross-correlation
ccn  = ac.ccn(data[0],mask=mask[0])

# compute average correlation of data parts 1 and 2
accn1 = AveragedAngularCorrelationMasked.from_dataset(data[:50],mask[:50])
accn2 = AveragedAngularCorrelationMasked.from_dataset(data[50:],mask[50:])

# combine partial results
accn1.merge(accn2)

# accessing mean and variance
accn1.mean
accn1.variance

[JamesWrigley]

Broadly looks fine, not that I'm an expert with XCCA 😛 Some thoughts:

• I would suggest putting AngularCorrelator and friends under applications, it feels too specific a thing to go in utils.
• The tests should pass.
• This should have a changelog entry.

[JamesWrigley]

Old docstring?

[JamesWrigley]

(also, please either rebase or do a squash merge 🙈 )

[codecov]

Codecov Report

❌ Patch coverage is 92.69103% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.82%. Comparing base (d94a426) to head (d3195ef).

Files with missing lines	Patch %	Lines
src/extra/applications/xcca.py	92.69%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
+ Coverage   72.97%   73.82%   +0.85%     
==========================================
  Files          35       36       +1     
  Lines        6642     6943     +301     
==========================================
+ Hits         4847     5126     +279     
- Misses       1795     1817      +22     

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[TimBerberich]

So, everything should be ready to merge now.

[JamesWrigley]

Needs a rebase to fix the changelog merge conflict, but otherwise LGTM!