No warning that variable for col parameter in mapping has to be nonnumeric.

[jschepers]

When running this code, col is just ignored because the values in the channel column are numeric. It would be nice if there was a warning/error in this case that says that the values have to be non-numeric.

plot_erp(effs;  mapping=(; y=:yhat,group= :channel,col=:channel))

[vladdez]

@behinger Is it fine if we just automatically convert values of :col to non-numeric?

[behinger]

should be, only issue is if one puts a numeric variable with many values by accident and then receives 1000 columns :D

[vladdez]

fixed #222