Allow user to suppress NA warning in count variables

[TimTaylor]

As part of improvements for #113 we now warn if a count column contains NA values (and nudge users to handle these before calling incidence():

https://github.com/reconverse/incidence2/blob/88c1fb9ecbe2ebe48973bf7815fc94451bc40e9f/R/incidence.R#L368-L375

We should provide arguments and options to suppress this if desired.