Add slope interval newton method

[eeshan9815]

@dpsanders Rebased "slope" and "debug-newton1d" branches onto this branch before adding more commits.

[eeshan9815]

AppVeyor build is failing on Windows 32-bit, one of the 3D roots tests is failing.

[dpsanders]

Could you rebase please?

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 0.78%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   64.97%   65.76%   +0.78%     
==========================================
  Files          11       11              
  Lines         551      552       +1     
==========================================
+ Hits          358      363       +5     
+ Misses        193      189       -4

Impacted Files	Coverage Δ
src/slopes.jl	83.33% <ø> (+4.76%)	:arrow_up:
src/IntervalRootFinding.jl	4.65% <ø> (ø)	:arrow_up:
src/newton1d.jl	87.38% <100%> (+0.11%)	:arrow_up:

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[dpsanders]

Another rebase required here...

[eeshan9815]

Done!

[dpsanders]

There's a real test error.

[eeshan9815]

Sorry that happened due to an error while resolving the merge conflicts. It should work fine now.

[dpsanders]

The function slope_newton1d should be refactored into the newton1d function, e.g. with a keyword arg.