Improve pretty printing

[lostella]

The following section of the documentation explains how to have separate behaviors for pretty printing objects and collections of objects. This is an issue in ProximalOperators, since pretty printing is very verbose and a tuple of proximable functions is displayed in a rather unreadable way: this makes me personally prefer not having any custom pretty printing at all, so I believe it could use some improvement.

https://docs.julialang.org/en/v1/manual/types/#man-custom-pretty-printing-1

• simplify show implementation (currently too verbose, with too many unmanageable details, like the domain)
• make sure a “compact” version is displayed when collections are displayed
• implement pretty printing for the types which are missing it

[lostella]

cc @nantonel @mfalt

[nantonel]

Definitively agree we should simplify this:

julia> using ProximalOperators

julia> NormL1()
description : weighted L1 norm
domain      : AbstractArray{Real}, AbstractArray{Complex}
expression  : x ↦ λ||x||_1
parameters  : λ = 1.0

We could have only the expression, something like:

julia> NormL1()
x ↦ 1.0 ||x||_1

or maybe even shorter:

julia> NormL1()
1.0 ||x||_1

[nantonel]

If we decide to completey avoid pretty printing, it works fine for some sfuff:

julia> NormL1()
NormL1{Float64}(1.0)

but it's quite horrible in other cases:

julia> IndAffine(randn(3,3),randn(3))
ProximalOperators.IndAffineDirect{Float64,Float64,Array{Float64,2},Array{Float64,1},LinearAlgebra.QRCompactWY{Float64,Array{Float64,2}}}([0.7540902122955551 -0.5127888696402579 -0.41036024039021046; 0.3041711542264574 -0.8821531613533968 0.3595632195466501; -0.6471214161239671 0.7621617642057313 0.018528841747891156], [-0.38375757488811657, -1.3589036717591345, 0.7437271582522681], LinearAlgebra.QRCompactWY{Float64,Array{Float64,2}}([-1.0 -0.5341803635161604 0.8864194955698212; -0.2923389378982839 0.8453705336914357 -0.46016377902426897; -0.23394477519669718 -0.3748487888220175 -0.05009764812690726], [1.754090212295555 0.6294821130997797 0.0; 5.0e-324 1.753598976639561 0.0; 2.297204718e-314 2.2972047183e-314 0.0]), [2.1888445724e-314, 2.1885385205e-314, 2.1885430936e-314])