DaqMX: create a generic/configurable DAQMX ioc

As an instrument scientist on muon beamlines, I sometimes need to monitor an external analogue voltage from various bits of kit (which might be user-supplied).

### Acceptance criteria

- [ ] A generic/configurable ioc & opi exist for talking to NI cDAQ units.
- [ ] Appropriate documentation on wiki

### Notes

- Requirement is for "slow" control & monitoring (e.g. ~1 read per second)
- A MOXA E12xx unit was suggested as an alternative to the scientists, but specifications appear to be unsuitable (unipolar and 16-bit reads)
- We already talk to cDAQ units in a number of other IOCs (`ZFMAGFLD`, `SEPARATOR`, `RKNPS`). However those iocs are tightly coupled to their specific situation. This ticket is to create a generic ioc for arbitrary external items.
- The immediate need is for analogue voltage readings, but consider whether we can also make this ioc handle digital inputs.