ID cannot be a factor, all character variables will be ignored

[romainkp]

Add a test that the ID variable provided by the user is NOT a factor within importData and return an error to the user otherwise.

Make it clear in the doc that the ID variable should be not be provided as a factor. Otherwise, the current implementation will create dummies for all study ID.

Make it clear in the doc that all character variables are ignored by stremr (i.e., they won't be converted to factors).