Adding median_first_quart_third_quart to pairwise_test_cont()

[lemireg]

A number of requests have come in for the testing comparison tables to include Median (25%, 75%) quartiles instead of the current output Median (Min, Max) as a part of the output of pairwise_test_cont() function.

I've started to draft the code to add this functionality (the name is too long! feel free to suggest another). Would be great to go over it with one of you or to get feedback. Thanks!

[lemireg]

Bryan's comments:

• you could get away with Group1_q1 instead of Group1_first_quart (and using q1 or q3 elsewhere)
• you should check in on the type argument for quantile() function and double-check what the standard is for SAS and hvtn. this is something we've run into in the past when trying to sync results. might be ok to keep the default.
• you might also force names = FALSE
• makes sure to update the tests too!