RIKENFE: Enable/Test NI Ethernet-GPIB communication with the Stanford DG535 Signal generator

As an instrument scientist I want to be able to use the DG535 signal generator to configure and read kicker timing values from the RIKENFE front end. It might also be useful to be able to use the DG645 as a swap in spare for this device (it is not currently known if there is a spare DG535)

### Acceptance Criteria
What is the acceptance criteria?
- [ ] An IOC (Possibly using the existing one for the DG645) can be used with the NI GPIB Ethernet adaptor to control this device.
- [ ] The communications are tested so that the decision to purchase more hardware can be made.

### Notes:
This is related to the more general RIKENFE ticket #2815 to provide IOC/OPI and communications with the DG535. It is more restricted in its aim which is to establish
a) That the GPIB Enet device can be used to talk to it (and the similar DG645 in case this is used as a spare).
b) Modify the the existing DG645 IOC (ticket #5154) to also work with the NI-VISA access on the basis that a general solution for both would be of immediate benefit to the muon teams