Ticks protocol

[heckj]

• collapsing TickScale protocol (& default implementations) into ContinuousScale
• adding DiscreteScale to match for Point and Band scale types
• added DiscreteScaleType to support CustomStringConvertible conformance
• added naive (wrong!) default implementation for ticks for discrete scales

[codecov]

Codecov Report

Merging #22 (2f580b2) into main (b2566e8) will increase coverage by 0.03%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   90.60%   90.63%   +0.03%     
==========================================
  Files          27       28       +1     
  Lines        2490     2552      +62     
==========================================
+ Hits         2256     2313      +57     
- Misses        234      239       +5     

Impacted Files	Coverage Δ
Sources/SwiftVizScale/AnyContinuousScale.swift	60.49% <ø> (+3.70%)	⬆️
Sources/SwiftVizScale/DiscreteScale.swift	0.00% <0.00%> (ø)
Sources/SwiftVizScale/LinearScale.swift	92.95% <ø> (ø)
Sources/SwiftVizScale/LogScale.swift	86.48% <ø> (ø)
Sources/SwiftVizScale/PowerScale.swift	88.46% <ø> (ø)
Sources/SwiftVizScale/RadialScale.swift	85.48% <ø> (ø)
Sources/SwiftVizScale/PointScale.swift	86.58% <80.00%> (-0.92%)	⬇️
Sources/SwiftVizScale/BandScale.swift	88.78% <83.33%> (-0.69%)	⬇️
Sources/SwiftVizScale/ContinuousScale.swift	83.52% <87.50%> (+5.15%)	⬆️
...s/SwiftVizScaleTests/AnyContinuousScaleTests.swift	100.00% <100.00%> (ø)
... and 5 more

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