Function to check multilevel models on the existence of random intercepts. The specification of an intercept by calling a 1-column (e.g. "int") is not counted towards the existence of an intercept. Contradictory inputs like "~ 1 + 0 + X1 + ..." or "~ -1 + 1 + X1 + ..." will throw an error.
random_intercept_check(model_formula)
A formula (from class formula
)
A boolean value indicating whether there is a fixed intercept in the model or not