Create radio button page helper

[remidefleurian]

Radio buttons can only be unselected, using selected = character(0) on the first iteration of a page. For further iterations, the previous answers are invisibly allocated if no answer is selected.

[remidefleurian]

See https://stackoverflow.com/questions/39535980/r-shiny-no-initial-value-selected-in-radiobuttons?rq=1

[pmcharrison]

Currently psychTestR does not explicitly support radio button pages. We should create a helper function for creating such pages, and then watch out for the issue you describe above.