Add support to read_tmo_ms for GPIBSerialCom.

This makes it possible to configure the time the Prologix GPIB<->USB yellow box waits on read for a response from the device before timing out. It uses the ++read_tmo_ms command (hence the option name) to set this. The default value in GPIBSerialCom is set to 0, meaning that no change is made.

I've seen a few cases where the ++read eoi returns an empty result because it was called before the device finished doing a measurement.