Create radiobuttons for reference groups to be selected in `create_probs_histogram()`

[uriahf]

• Create radiobuttons dynamically that extract the reference groups and produces radiobuttons.
  These radiobuttons must be available in the crosstalk object, so it should be vailable in both summary report and in extnal viewer.
  This code already exists in

  rtichoke/R/functions_for_crosstalk.R

  Line 4 in 09428c2

  group,

  
  but it should be generic as possible

Originally posted by @uriahf in #139