filterPD-methods: Simple filtering of a partial discharge signal
In oscarperpinan/pdcluster: Partial Discharge Clustering

Filtering of a partial discharge signal.

## S4 method for signature 'PD'
filterPD(object,
         filter=N>10 & nZC>0 & W4>0 & freq1>1 & freq1<2e7 & !refl,
         select)

`object`	A `PD` or `PDCluster` object.
`filter`	logical expression indicating elements to keep. This expression will be evaluated in the `data` slot of the `object` (as in `subset`).
`select`	Variables to be selected. This expression will be evaluated in the `data` of the object (as in `subset`).