Rewrite GreenFunction plotting

[pablosanjose]

Closes #250

The old graphical representation of GreenFunctions was quite lacking and confusing.

This implements a new way of plotting GreenFunctions that includes boundaries, self-energy sites and sites/couplings of children Hamiltonians coupled to said sites. The latter are shown with squares/cubes instead of circles/spheres.

An example

julia> glead = LP.honeycomb() |> hopping(1, range = 1) |> supercell((1,-1), region = r -> 0<=r[2]<=5) |> attach(nothing, cells = SA[5]) |> greenfunction(GS.Schur(boundary = 0));

julia> g = LP.honeycomb() |> hopping(1) |> supercell(region = r -> -6<=r[1]<=6 && 0<=r[2]<=5) |> attach(glead, region = r -> r[1] > 4.9) |> greenfunction;

julia> qplot(g, shellopacity = 0.3)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.35294% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 92.07%. Comparing base (624c354) to head (139364f).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/solvers/selfenergy/schur.jl	66.66%	9 Missing ⚠️
src/solvers/green/bands.jl	0.00%	2 Missing ⚠️
src/types.jl	91.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   92.16%   92.07%   -0.10%     
==========================================
  Files          35       36       +1     
  Lines        5823     5857      +34     
==========================================
+ Hits         5367     5393      +26     
- Misses        456      464       +8     

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