Add frequency estimators by Jacobsen and Quinn

[mbaz]

This commit adds two new frequency estimation algorithms:

* `jacobsen`, a very fast algorithm that performs a three-point
  curve fit around the DFT peak. Not super accurate.
* `quinn`, a very accurate iterative algorithm based on ARMA(2,2).
  It requires an initial frequency estimate (which may be
  provided by `jacobsen`).

Both algorithms work with real and complex signals.

[ViralBShah]

Is this good to merge?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.63%. Comparing base (10a7c1e) to head (64058c6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #503      +/-   ##
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+ Coverage   97.58%   97.63%   +0.05%     
==========================================
  Files          18       18              
  Lines        3147     3216      +69     
==========================================
+ Hits         3071     3140      +69     
  Misses         76       76              

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[mbaz]

Added a couple of tests to include the two lines that were missing coverage.

[ViralBShah]

Perhaps good to wait for a bit for @martinholters to take a look.

[mbaz]

@martinholters Thank you for the review!

[ViralBShah]

@mbaz In preparation for a 0.8 release, would it be possible for you to get this PR ready?

[mbaz]

@ViralBShah I'm still actively working on it. Two things happened: (1) I realized applying Jacobsen to real signals requires a bit of care, and (2) my work load peaked the past two weeks.

My plan at the moment is to have a single Jacobsen estimator based on the DFT. If/when I work out the issues with an RFFT-based estimator, I'll make another PR. How does that sound?

What is the timeline for 0.8? I will try to finish up this PR in the next couple of days.

[ViralBShah]

Thank you. There is no tight deadline, and we can always add new features in new releases. I am just thinking out aloud, but if we can get as much of the backlog of PRs in as we can in the next 2 weeks or so - that will make for a nice 0.8 release.

@martinholters I would of course defer to you on that.

[martinholters]

Agree on the 0.8 release. Would be nice to get of bunch of things done for it which are (and some of them have been for while) in an almost-ready state. OTOH, I don't want to release 0.8 too hastily in case we discover something else that requires a breaking change. My gut feeling is that 2 to 4 weeks from now might be a good time.

[ViralBShah]

@mbaz Just a gentle nudge to see if possible to finish this PR. 0.8 may be in a week or two. But of course, if you are busy, there's always 0.9!

[mbaz]

The test failure seems to be related to codecov.

[mbaz]

Thank you too, @martinholters -- the code is much better now than it was in the original PR.