tick updates

[heckj]

• added methods to get default tick values just in terms of the strings
• updated tick methods to pass through a formatter, if provided - defaulting to nil
• added methods on Point and Band scales to get lists of strings for their ticks

[codecov]

Codecov Report

Merging #23 (a3b0196) into main (5a01ca3) will decrease coverage by 0.85%.
The diff coverage is 69.00%.

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   90.63%   89.77%   -0.86%     
==========================================
  Files          28       28              
  Lines        2552     2622      +70     
==========================================
+ Hits         2313     2354      +41     
- Misses        239      268      +29     

Impacted Files	Coverage Δ
Sources/SwiftVizScale/DiscreteScale.swift	0.00% <ø> (ø)
Sources/SwiftVizScale/LinearScale.swift	92.95% <ø> (ø)
Sources/SwiftVizScale/ContinuousScale.swift	67.76% <48.07%> (-15.77%)	⬇️
Sources/SwiftVizScale/BandScale.swift	87.61% <75.00%> (-1.18%)	⬇️
Sources/SwiftVizScale/PointScale.swift	85.22% <75.00%> (-1.36%)	⬇️
Sources/SwiftVizScale/NiceValue.swift	94.98% <100.00%> (ø)
...ests/SwiftVizScaleTests/ExternalPackageTests.swift	100.00% <100.00%> (ø)
Tests/SwiftVizScaleTests/LinearScaleTests.swift	87.61% <100.00%> (ø)
Tests/SwiftVizScaleTests/LogScaleTests.swift	86.29% <100.00%> (ø)
Tests/SwiftVizScaleTests/TickTests.swift	86.66% <100.00%> (+4.31%)	⬆️

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