Adding indicator of polyhedral set

[lostella]

This adds the indicator of a polyhedral set

{x : xmin <= x <= xmax, l <= Ax <= u}

For the moment, the projection is only implemented using OSQP. This PR is backward compatible in that it just adds a function. The only additional change is a minor modification to a test routine.

[lostella]

Relevant for #52

[codecov]

Codecov Report

Merging #54 into master will decrease coverage by 0.15%.
The diff coverage is 97.22%.

@@           Coverage Diff            @@
##           master    #54      +/-   ##
========================================
- Coverage   92.16%    92%   -0.16%     
========================================
  Files          67     69       +2     
  Lines        1991   2039      +48     
========================================
+ Hits         1835   1876      +41     
- Misses        156    163       +7

Impacted Files	Coverage Δ
src/ProximalOperators.jl	100% <ø> (ø)	⬆️
src/functions/indPolyhedralOSQP.jl	100% <100%> (ø)
src/functions/indPolyhedral.jl	80% <80%> (ø)
src/functions/indHalfspace.jl	76% <0%> (-20%)	⬇️
src/calculus/translate.jl	52% <0%> (-4%)	⬇️
src/functions/indExp.jl	93.42% <0%> (-3.95%)	⬇️
src/functions/linear.jl	38.09% <0%> (+0.59%)	⬆️
src/functions/logisticLoss.jl	90.24% <0%> (+2%)	⬆️
... and 2 more

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