bug fix for get_mu_and_sigma, IPCA_Params now exposed, contrast curve docstring fix

[IainHammond]

• Fixed a critical mistake in get_mu_and_sigma where planet separation was used instead of planet theta in the 4D case
• The IPCA_Params in ipca is now exposed and available for use. This was to fix a bug but I can't remember how I triggered it. Just trust me
• Fixed some outdated descriptions in the contrast_curve docstring

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.23%. Comparing base (510e7f3) to head (b48166e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/vip_hci/fm/negfc_fmerit.py	0.00%	1 Missing ⚠️
src/vip_hci/greedy/ipca_fullfr.py	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #732   +/-   ##
=======================================
  Coverage   50.23%   50.23%           
=======================================
  Files          88       88           
  Lines       17089    17089           
=======================================
  Hits         8584     8584           
  Misses       8505     8505           

Flag	Coverage Δ
no_objects	49.05% <0.00%> (ø)
objects	60.43% <ø> (ø)

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