Add `PECInsulator` boundary condition

[dpgrote]

This PR adds a mixed PEC and insulator boundary condition. This allows an insulator to be placed on a portion of the boundary. The rest of that boundary will be PEC. Within the insulator portion, the tangential fields can be specified on the boundary (as functions of space and time). The normal fields and fields not specified are extrapolated to the guard cells from the valid cells. The fields are specified in pairs, the two tangential electric fields, and the two tangential magnetic fields. In each pair, if one is set, the other will be zeroed if not set.

A use case is the simulation of a dynamic pinch, driven by an external current, represented as a time dependent B field on the boundary.