implement boundary and normal of domain

[daanhb]

This is a work in progress to address #83

[codecov]

Codecov Report

Merging #84 (4e40045) into master (31b29a2) will decrease coverage by 1.39%.
The diff coverage is 71.62%.

❗ Current head 4e40045 differs from pull request most recent head 6716483. Consider uploading reports for the commit 6716483 to get more accurate results

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
- Coverage   86.20%   84.80%   -1.40%     
==========================================
  Files          29       30       +1     
  Lines        2196     2376     +180     
==========================================
+ Hits         1893     2015     +122     
- Misses        303      361      +58     

Impacted Files	Coverage Δ
src/DomainSets.jl	100.00% <ø> (ø)
src/applications/coordinates.jl	0.00% <0.00%> (ø)
src/domains/indicator.jl	93.54% <0.00%> (-3.12%)	⬇️
src/domains/trivial.jl	92.98% <0.00%> (-7.02%)	⬇️
src/maps/composite.jl	64.61% <0.00%> (-0.51%)	⬇️
src/domains/simplex.jl	67.90% <3.70%> (-32.10%)	⬇️
src/domains/levelset.jl	95.58% <25.00%> (-4.42%)	⬇️
src/maps/affine.jl	82.40% <33.33%> (-0.60%)	⬇️
src/domains/interval.jl	94.41% <38.46%> (-4.39%)	⬇️
src/domains/ball.jl	97.32% <57.14%> (-1.11%)	⬇️
... and 12 more

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