turn off smooth if there are too few unique values for splines

[EmilHvitfeldt]

To close #155

This change happens if the number of unique within any of the groups is too low. To avoid a complicated "some groups use splines some do not" structure that would need to be added

[topepo]

Thanks! I made some changes to work with the refactored version. For example, we are not bothering with grouped data frames, estimate is a character vector, tidyselect happens early, etc.