Add bisection root-finder

[dpsanders]

Fixes #204.

This adds a bisection root finder that works with single or multiple variables.

Currently, this involves modifying the Root type.
For bisection in particular, this is, in some sense, not necessary, since it cannot prove uniqueness, so any roots are :unknown.

However, for multi-dimensional root finding in general, it is maybe necessary to do so.

[coveralls]

Coverage decreased (-1.07%) to 89.194% when pulling cedefba on bisection into 1c0b48f on master.

[codecov-io]

Codecov Report

Merging #262 into master will decrease coverage by 1.31%.
The diff coverage is 52.38%.

@@           Coverage Diff            @@
##           master   #262      +/-   ##
========================================
- Coverage   90.31%    89%   -1.32%     
========================================
  Files          26     27       +1     
  Lines        1084   1100      +16     
========================================
  Hits          979    979              
- Misses        105    121      +16

Impacted Files	Coverage Δ
src/root_finding/bisection.jl	0% <0%> (ø)
src/root_finding/newton.jl	80.7% <100%> (-10.53%)	⬇️
src/root_finding/root_finding.jl	86.79% <100%> (+1.68%)	⬆️
src/root_finding/krawczyk.jl	95.45% <100%> (ø)	⬆️

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[coveralls]

Coverage decreased (-1.07%) to 89.194% when pulling cedefba on bisection into 1c0b48f on master.

[coveralls]

Coverage decreased (-1.07%) to 89.194% when pulling cedefba on bisection into 1c0b48f on master.

[coveralls]

Coverage decreased (-1.07%) to 89.194% when pulling 71d24b4 on bisection into 1c0b48f on master.

[coveralls]

Coverage decreased (-1.3%) to 89.0% when pulling e42821e on bisection into ac8d0a8 on master.

[dpsanders]

This is ready. Any objection to merging, @lbenet ?

[dpsanders]

Moved to JuliaIntervals/IntervalRootFinding.jl#7