ENH: 4DD extra volume prep

Adding the ability to remove poor gates based on the normalized coherent power and cross correlation ratio fields, assuming `prep = 1`. This is added (quite easily) using an additional kwarg called `extra_prep`. This should further help the 4DD algorithm by removing noise from the receiver and second-trip echoes. I decided to add this feature as I noticed the `lowdbz` and `highdbz` thresholds for the reflectivity field were not adequately removing enough bad gates, especially those representing second-trip echoes.