WIP: Implement rounding for Intervals

[omus]

Allows uses to round intervals. I've had this idea as a feature for a while but the real trick is deciding what endpoints to round. For Intervals type we can do both endpoints or just one while with AnchoredIntervals we can only do one since doing both could result a different type being returned.

Additions needed yet:

• Tests for rounding AnchoredIntervals
• Documenting on keyword used by rounding functions
• Document Endpoint (Document LeftEndpoint and RightEndpoint #28)

[codecov]

Codecov Report

Merging #65 into master will decrease coverage by 3.61%.
The diff coverage is 62.96%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   97.89%   94.27%   -3.62%     
==========================================
  Files           6        6              
  Lines         237      262      +25     
==========================================
+ Hits          232      247      +15     
- Misses          5       15      +10

Impacted Files	Coverage Δ
src/endpoint.jl	96.66% <100%> (+0.23%)	⬆️
src/anchoredinterval.jl	85.5% <35.71%> (-12.74%)	⬇️
src/interval.jl	97.32% <90.9%> (-0.7%)	⬇️

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