More VegaLite plot directives

[pablosanjose]

In preparation for #103, we add the ability to specify (site-dependent) opacity, size and color of sites and links in VegaLite plots of Hamiltonians. The vlplot keywords are now:

    - `size = 800`: the `(width, height)` of the plot (or `max(width, height)` if a single number)
    - `points = false`: whether to plot points on line plots
    - `labels`: labels for the x and y plot axes. Defaults `("φ/2π", "ε")` and `("x", "y")` respectively
    - `scaling = (1/2π, 1)`: `(scalex, scaley)` scalings for the x (Bloch phase) and y (energy) variables
    - `xlims = missing` and `ylims = missing`: `(xmin, xmax)` and `(ymin, ymax)` to constrain plot range
    - `bands = missing`: bands to plot (or all if `missing`)
    - `axes = (1,2)`: lattice axes to project onto the plot x-y plane
    - `digits = 4`: number of significant digits to show in onsite energy and hopping tooltips
    - `plotsites = true`: whether to plot sites
    - `plotlinks = true`: whether to plot links
    - `sitesize = 15`: diameter of sites in pixels. Can be a function of site index.
    - `siteopacity = 0.9`: opacity of sites. Can be a function of site index.
    - `linksize = 0.25`: thickness of hopping links as a fraction of sitesize. Can be a function of site indices.
    - `linkopacity = 1.0`: opacity of hopping links. Can be a function of site indices.
    - `sitecolor = missing`: function of site index that returns a real to be enconded into a color, using `colorscheme`
    - `linkcolor = sitecolor`: function of link indices that returns a real to be enconded into a color, using `colorscheme`
    - `colorscheme`: Color scheme from `https://vega.github.io/vega/docs/schemes/` to be used (defaults "category10" or "lightgreyred")

[codecov-io]

Codecov Report

Merging #110 into master will decrease coverage by 0.45%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   61.58%   61.13%   -0.46%     
==========================================
  Files          16       16              
  Lines        2705     2725      +20     
==========================================
  Hits         1666     1666              
- Misses       1039     1059      +20     

Impacted Files	Coverage Δ
src/plot_vegalite.jl	0.00% <0.00%> (ø)

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