Scale.{xy}group documentation

[Mattriks]

• I've updated the documentation to reflect these changes
• 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:

• adds documentation for Scale.xgroup() and Scale.ygroup()
• closes Order of Geom.subplot_grid #496

[codecov-io]

Codecov Report

Merging #1247 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1247   +/-   ##
======================================
  Coverage    85.6%   85.6%           
======================================
  Files          35      35           
  Lines        4112    4112           
======================================
  Hits         3520    3520           
  Misses        592     592

Impacted Files	Coverage Δ
src/scale.jl	94.51% <ø> (ø)	⬆️

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