Merge and filter formulas. This function can be used to merge the formulas*.RDS created by different runs of `OmicSelector_OmicSelector()` and filter them to keep the maximum number of miRNAs. This may be useful in planning qPCR validation. The result of this function is `featureselection_formulas_final.RDS` file, which can be futher supplied to `OmicSelector_benchmark()`.