Informative error when tran names are mixed case in as_data_set()

[kylebaron]

Summary

In #1196, we refactored how as_data_set() deals with data frame, ev and evd objects.

When an input object is like this

data.frame(time = 0, RATE = 5, rate = 2)

We decide to go with lower case naming scheme and RATE becomes rate.1 and you'll get a warning for missing values in the final object (no missing values warning if all objects are like this, but you'll still get rate.1).

Let's do another pass where we check for mixed names and give a more specific warning sooner.