new symbol: Conn_Coaxial_x2_Isolated

the existing Conn_Coaxial_x2 connects the outer pins of both connectors together, this is not the case for isolated connectors (e.g. for BNC)

It is a copy of the existing Conn_Coaxial, converting it into a two part unit with two identical components. As this is a generic connector there is no datasheet linked, an example component is this. A footprint will follow in the near future.