colourscheme dropdown for exp plot

[hasaru-k]

• the standard colourscheme for the expression plot is category10 unless sample.cols is supplied, in which case the colour scheme is whatever the user gives
• this PR changes this behaviour so that:
  • a dropdown menu of discrete colourschemes appears when sample.cols is not provided (with category10 as the first option)
  • otherwise, nothing appears

[codecov-io]

Codecov Report

Merging #59 (4eb4940) into master (42dde80) will increase coverage by 3.56%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   90.50%   94.06%   +3.56%     
==========================================
  Files           5        5              
  Lines         337      337              
==========================================
+ Hits          305      317      +12     
+ Misses         32       20      -12     

Impacted Files	Coverage Δ
R/glimmaXY.R	96.15% <0.00%> (+15.38%)	:arrow_up:

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