rework the Tracks object; close #278, close #277

[jverzani]

Following #278 this reworks the Tracks structure:

• rather than use xs and fs to store values; this uses xfs and abs to store values depending on if the method is bracketing or not
• this needs improved documentation
• Could the interface of ValueHistories be useful here?
• This also addresses part of issue Initial iterations for Bisection and ITP do work correctly #279 (ITP is corrected), but the initial step for Bisection is lost
• this would close Tracker works poorly for bracketing methods #277 too?
• Not sure about the suggestion in Tracks is a bad name #276

[codecov]

Codecov Report

Merging #280 (e3d7e29) into master (feb9bee) will increase coverage by 0.12%.
The diff coverage is 56.06%.

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   84.51%   84.64%   +0.12%     
==========================================
  Files          12       12              
  Lines        2086     2084       -2     
==========================================
+ Hits         1763     1764       +1     
+ Misses        323      320       -3     

Impacted Files	Coverage Δ
src/find_zeros.jl	95.90% <ø> (ø)
src/bracketing.jl	90.41% <35.29%> (+1.19%)	⬆️
src/find_zero.jl	70.12% <53.84%> (-1.55%)	⬇️
src/order0.jl	89.00% <100.00%> (+0.34%)	⬆️

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[KronosTheLate]

I want to mention that NLSolve.jl uses the three kwargs show_trace::Bool, store_trace::Bool, and extended_trace::Bool. Trace feels very natural in my mind, and is in fact the word that comes to mind when I think about this. Consistency within the ecosystem is something that I find extremely valuable.

[jverzani]

Let's make the name a separate issue, as it will be breaking. This PR should improve the structure now known as Tracks, as it has the benefit of working better with the hybrid Order0 method and other tweaks. Once I merge this PR, would you port your documentation improvements, with any adjustment? (I could also copy and paste them in to this PR if you want, just let me know).

[KronosTheLate]

Let's make the name a separate issue, as it will be breaking. This PR should improve the structure now known as Tracks, as it has the benefit of working better with the hybrid Order0 method and other tweaks. Once I merge this PR, would you port your documentation improvements, with any adjustment? (I could also copy and paste them in to this PR if you want, just let me know).

Good idea keeping it separate. I see no problem in you copying in any changes I made in my PR - remember to include the new section in the docs ^_^