add projection methods

[codecov[bot]]

Codecov Report

Merging #5 into master will increase coverage by 8.25%. The diff coverage is 86.45%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   67.18%   75.44%   +8.25%     
==========================================
  Files           3        4       +1     
  Lines         128      224      +96     
==========================================
+ Hits           86      169      +83     
- Misses         42       55      +13     

Impacted Files	Coverage Δ
src/MathOptSetDistances.jl	100.00% <ø> (ø)
src/distances.jl	50.00% <0.00%> (-25.00%)	:arrow_down:
src/sets.jl	66.66% <ø> (ø)
src/projections.jl	88.29% <88.29%> (ø)

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[matbesancon]

The general rule for which distance to implement: whenever you can

1. implement one of the specific distances
2. make the DefaultDistance fall back to the most "obvious" one

[matbesancon]

Fixes #4

[matbesancon]

A good property to test for all sets: The distance_to_set of the projection should always be 0

[matbesancon]

the docstring changes can be applied everywhere (in all docstrings)

[matbesancon]

Overall, I am not sure the dual argument is necessary, if people need the projection on the dual, they can just project(dist, z, dual(s))

[matbesancon]

this will also simplify some functions IMO

[matbesancon]

bump :)

[matbesancon]

See codecov comments for where to add tests

[akshay326]

add test for projections and gradients

[akshay326]

actually this was useful: found and fixed a bug in one of the projection methods