Support multiple states in vlplot

[pablosanjose]

When passing a multiply degenerate subspace s::Subspace or a matrix s::AbstractMatrix to vlplot(h, s; shaders...), each basis vector or column of s will now be summed over after applying shaders to each. This allows to visualize observables integrated over a whole subspace (e.g. summing local density over all spin flavors).

[codecov-io]

Codecov Report

Merging #140 (aa31be2) into master (8332f15) will decrease coverage by 0.08%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
- Coverage   61.03%   60.94%   -0.09%     
==========================================
  Files          16       16              
  Lines        2972     2978       +6     
==========================================
+ Hits         1814     1815       +1     
- Misses       1158     1163       +5     

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% <69.23%> (-0.31%)	⬇️

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