Size revamp 1: Scale.size_discrete2

[Mattriks]

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• First in a set of 3 PRs. This PR adds Scale.size_discrete2 in the style of Scale.color_discrete.
  Scale.size_discrete can be switched to the new function e.g. in Gadfly 2.0. Until then, Scale.size_discrete2 can be used.
• Scale.size_discrete2(f=default_discrete_sizes), f is a function which maps from categorical values to Measure units, and allows arbitrary mappings.
• default_discrete_sizes has the same range as currently provided by Theme(point_size_min, point_size_max)
• Theme(discrete_sizemap=default_discrete_sizes) replaces Theme(point_size_min, point_size_max), which can be deprecated.
• The 3 PRs will contain some functions to be moved to Measures.jl, but I won't do that until the third PR.

Example

This PR fixes the problem mentioned in #1358 with plot p2. Example:

y = rand(5)
pcurrent = plot(x=1:5, y=y, size=["C", "A", "A", "B", "C"],
     Theme(point_size_min=4pt, point_size_max=16pt) )
     
pnew = plot(x=1:5, y=y, size=["C", "A", "A", "B", "C"], 
    Scale.size_discrete2(n->range(4pt, 16pt, length=n)) )
    
hstack(pcurrent, pnew)

In pcurrent, the y-axis is expanded by ±3 because Scale.size_discrete maps size to Integers (1:3). The mapping of the size aesthetic to size units needs to happen earlier, as in pnew, with Scale.size_discrete2

[codecov-io]

Codecov Report

Merging #1376 into master will increase coverage by 0.01%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #1376      +/-   ##
==========================================
+ Coverage   90.55%   90.57%   +0.01%     
==========================================
  Files          38       39       +1     
  Lines        3961     3988      +27     
==========================================
+ Hits         3587     3612      +25     
- Misses        374      376       +2

Impacted Files	Coverage Δ
src/theme.jl	69.49% <ø> (ø)	⬆️
src/scale.jl	97.41% <ø> (ø)	⬆️
src/aesthetics.jl	81.51% <ø> (ø)	⬆️
src/geom/point.jl	100% <100%> (ø)	⬆️
src/scale/scales.jl	92.59% <92.59%> (ø)

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