Fix codecov not reporting

[itrharrison]

Hopefully this is just adding the output of coverage to an xml file which is explicitly required bby the new uploader version. Merging will close #102

[codecov-commenter]

Codecov Report

Merging #103 (9bd00e9) into master (b21cbc7) will decrease coverage by 14.29%.
The diff coverage is 68.35%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #103       +/-   ##
===========================================
- Coverage   76.00%   61.72%   -14.29%     
===========================================
  Files          29       30        +1     
  Lines        1817     1941      +124     
===========================================
- Hits         1381     1198      -183     
- Misses        436      743      +307     

Impacted Files	Coverage Δ
soliket/cosmopower.py	68.94% <65.51%> (-31.06%)	⬇️
soliket/__init__.py	100.00% <100.00%> (ø)
soliket/cross_correlation.py	99.20% <100.00%> (ø)
soliket/gaussian.py	69.73% <100.00%> (+0.40%)	⬆️
soliket/gaussian_data.py	87.00% <100.00%> (+0.54%)	⬆️
soliket/mflike/mflike.py	11.00% <0.00%> (-75.00%)	⬇️
soliket/foreground.py	42.22% <0.00%> (-53.34%)	⬇️
soliket/mflike/theoryforge_MFLike.py	37.93% <0.00%> (-48.28%)	⬇️
soliket/bandpass.py	27.55% <0.00%> (-17.35%)	⬇️