Correlator maintainance

[jngrad]

Description of changes:

• Remove documentation of nonexistent correlator complex_conjugate_product
• Remove unused, undocumented and untested correlator get_correlation_time
• Update documentation of the tau_lin parameter and FCS correlator
• Add testcase for the FCS correlator and the linear tau correlator

[review-notebook-app]

Check out this pull request on 

Review Jupyter notebook visual diffs & provide feedback on notebooks.

---

Powered by ReviewNB

[codecov]

Codecov Report

Merging #3783 into python will increase coverage by 0%.
The diff coverage is 90%.

@@          Coverage Diff           @@
##           python   #3783   +/-   ##
======================================
  Coverage      88%     89%           
======================================
  Files         552     552           
  Lines       24446   24421   -25     
======================================
+ Hits        21714   21735   +21     
+ Misses       2732    2686   -46     

Impacted Files	Coverage Δ
src/core/accumulators/Correlator.hpp	90% <ø> (ø)
src/core/accumulators/Correlator.cpp	85% <90%> (+13%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9cc0cd1...b59a527. Read the comment docs.

[fweik]

LGTM