Keithley 2000: add arbitrary command functionality

## Where?

As a developer or electronics technician, I would like the ability to easily send an arbitrary command to a Keithley model 2000 via IBEX, so that "unusual" setup commands can be easily supported.

We have an "arbitrary command" mechanism on several other devices (e.g. Keithley 2400), the 2000s have a lot of functionality that we haven't exposed via EPICS, an arbitrary command PV would let us or samp env technicians set these parameters without killing the IOC and using `hterm`, and would act as a fallback in case users need to access one of these unusual parameters from a script.

## Acceptance criteria
- [ ] Arbitrary command mechanism - comparable to existing mechanism in Keithley 2400 - added to Keithley 2000 ioc

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_