Allow filtering with implicit and semi-implicit solver

[dpgrote]

This PR adds a call to filter the E-fields that are applied to the particles when filtering is turned on. The current is already filtered in the call to SyncCurrentAndRho. With the filtering of E, the energy conservation is maintained.

Note that it is using the ApplyFilterJ routine to apply the filtering to the E-field. Perhaps that routine could be renamed to ApplyFilterMF since it is not specific to J.