Cubic L2 norm, typing fixes, tests cleanup

[lostella]

Partially addresses #58 by adding cube Euclidean norm and tests for it, cleans some of the existing functions (in the typing and type parameters mostly), improves some of the existing tests.

[codecov]

Codecov Report

Merging #63 into master will decrease coverage by 0.41%.
The diff coverage is 77.5%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   60.94%   60.52%   -0.42%     
==========================================
  Files          70       71       +1     
  Lines        2038     2052      +14     
==========================================
  Hits         1242     1242              
- Misses        796      810      +14

Impacted Files	Coverage Δ
src/ProximalOperators.jl	48% <ø> (ø)	⬆️
src/functions/sqrNormL2.jl	76% <ø> (ø)	⬆️
src/functions/indBallL0.jl	65.51% <100%> (ø)	⬆️
src/functions/indHyperslab.jl	84% <100%> (ø)	⬆️
src/functions/indBallL1.jl	51.16% <62.5%> (ø)	⬆️
src/functions/cubeNormL2.jl	68.75% <68.75%> (ø)
src/calculus/conjugate.jl	57.69% <87.5%> (-3.03%)	⬇️
src/functions/normL1.jl	55.07% <0%> (-11.6%)	⬇️
src/functions/linear.jl	66.66% <0%> (-6.67%)	⬇️

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