Add support for ethernet-based serial adaptor

[alexdewar]

The UNIRAS rig will use an ethernet-based serial adaptor, CSC-H64 by Sollae Systems, in place of the current USB serial adaptors. This will require some (small) modifications to the existing code, which currently only works with USB serial adaptors.

There are two ways we could approach this:

1. The serial adaptor comes with software which creates virtual COM ports that can be used as regular serial devices. This option would require that we change the code to work with COM ports of any type, rather than just those associated with USB devices.
2. We may be able to communicate with the serial adaptor directly, without any additional software required. There is an open standard for serial over IP connections (RFC2217), which is supported by the Python library we are using for serial connectivity (PySerial) and, hopefully, the serial adaptor will support this protocol. (If not, we may still be able to make it work ourselves, but in this case, it is likely that the previous option will be better.) One advantage of this approach is that we won’t have to hardcode COM-port numbers (which are somewhat fragile) into our config files.

[alexdewar]

Marking as "on hold" until we actually have one of these adaptors to play with.