Lensing can read sacc and fiducial file

[ggalloni]

This PR fixes #60

• Possibility to read the fiducial from a file.
• Reading the correction factors from a sacc file.
• @mgerbino I am not sure about the "move theory calculations" point. Do you have any suggestions?

Also,

• Probably we should modify the installation files

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.88%. Comparing base (52be7b3) to head (f0ed463).

Files with missing lines	Patch %	Lines
soliket/lensing/lensing.py	40.00%	28 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   74.10%   72.88%   -1.22%     
==========================================
  Files          35       35              
  Lines        1641     1678      +37     
  Branches      146      149       +3     
==========================================
+ Hits         1216     1223       +7     
- Misses        371      399      +28     
- Partials       54       56       +2     

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[mgerbino]

@ggalloni, I believe we can skip that part. It was intended to mirror the "old" structure of mflike, when everything that wasn't data preparation and loglike computation was included in a separate module (theoryforge). Since that structure is no longer in place, there is no need to implement it here. Thanks for checking!