adding IndHyperslab, fixing IndHalfspace and Linear

[lostella]

This:

• adds IndHyperslab, partially addressing Missing functions and calculus rules #58 (very partially)
• fixes IndHalfspace in that the improper case is detected, and AbstractArrays are allowed instead of just AbstractVectors
• fixes Linear in that only real coefficients are allowed
• makes parametric types a bit cleaner in all of the above

[codecov]

Codecov Report

Merging #62 into master will decrease coverage by 36.26%.
The diff coverage is 85%.

@@             Coverage Diff             @@
##           master      #62       +/-   ##
===========================================
- Coverage   96.79%   60.53%   -36.27%     
===========================================
  Files          68       70        +2     
  Lines        1248     2037      +789     
===========================================
+ Hits         1208     1233       +25     
- Misses         40      804      +764

Impacted Files	Coverage Δ
src/ProximalOperators.jl	48% <ø> (-52%)	⬇️
src/functions/linear.jl	73.33% <ø> (-26.67%)	⬇️
src/functions/indHyperslab.jl	84% <84%> (ø)
src/functions/indHalfspace.jl	62.5% <86.66%> (-4.17%)	⬇️
src/functions/indFree.jl	27.77% <0%> (-72.23%)	⬇️
src/utilities/linops.jl	40% <0%> (-60%)	⬇️
src/calculus/tilt.jl	42.1% <0%> (-57.9%)	⬇️
src/calculus/translate.jl	44% <0%> (-56%)	⬇️
src/calculus/postcompose.jl	46.42% <0%> (-53.58%)	⬇️
... and 60 more

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[lostella]

Coverage drop due to JuliaCI/Coverage.jl#187