Fix in vlplot: make CurrentShader and DensityShader work with multiple states

[pablosanjose]

This completes #140 by adding support for multiple states in current and density shaders. Thanks to @fernandopenaranda for the heads up!

This PR also tries to make CurrentShader more correct in open systems. In doing so I realized we have a serious problem here: we cannot compute currents across unit cell boundaries from the Bloch eigenstate alone. We also need its Bloch phases. Solving this issue requires some thought about design (e.g. what information should we store and provide about a generic eigenstate?), so I do not attempt to fix that here. For the moment, therefore, be aware that current density plots is not going to be correct in open systems.

This also introduces a stopgap fix for a problem also reported by @fernandopenaranda:

julia> h = LatticePresets.honeycomb() |> hamiltonian(hopping(I)) |> unitcell(10, region = !RP.circle(2, (0,6)));

julia> bs = bandstructure(h, subticks = 13);
Step 1/2 - Diagonalising: 100%|███████████████████████████████████████████████████████████████████████████████████████████████████| Time: 0:00:01
Step 2/2 - Knitting bands: 100%|██████████████████████████████████████████████████████████████████████████████████████████████████| Time: 0:00:02
ERROR: BoundsError: attempt to access 12×12 Array{UnitRange{Int64},2} at index [13, 1]

It it not worthwhile to dig deeper into this issue to make a better fix than the one included here, as there is a pending refactor of the bandstructure machinery that will make the corresponding code obsolete

[codecov-io]

Codecov Report

Merging #141 (5433953) into master (98016ec) will increase coverage by 0.10%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   61.03%   61.13%   +0.10%     
==========================================
  Files          16       16              
  Lines        2972     2972              
==========================================
+ Hits         1814     1817       +3     
+ Misses       1158     1155       -3     

Impacted Files	Coverage Δ
src/plot_vegalite.jl	0.00% <0.00%> (ø)
src/tools.jl	36.84% <0.00%> (-0.17%)	⬇️
src/hamiltonian.jl	80.00% <66.66%> (-0.31%)	⬇️
src/bandstructure.jl	81.88% <100.00%> (+0.09%)	⬆️

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