This function parses a obs_filter argument passed through a predict_...
function into a string ready to be evaluated in merge_prediction(). The output
string will be parsed within a case_when. The obs_filter passed sets a
threshold for fitting models and generating predictions for data based on
number of observed data points. So, '>=5' would be that we only want to fit a model,
generate predictions and use them them to replace observations
if there are 5 or more observations. Thus, in case_when(), we pass the first
logical argument to be anywhere there is NOT '>=5' observations.