Add excitations logic to lakeshore 340 IOC

[JamesKingWork]

Description of work

Add macros and to set and get file correctly.

To test

ISISComputingGroup/IBEX#5950

Acceptance criteria

List the acceptance criteria for the PR

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Code Review

• A copy of the manual has been placed on the shared drive
• Pertitent information has been stored in the wiki
• Does the IOC conform to IBEX standards?
  • PV naming.
  • Disable records
  • Record simulation
  • Finishing touches
• If an OPI has been modified, does it conform to the style guidelines?
• Do the IOC and support module conform to the new build guidelines
• Have the changes been recorded appropriately in a PR for release notes?
• Is the device's flow control setting correct? For most devices flow control should be OFF.

Functional Tests

• IOC responds correctly in:
  • Devsim mode
  • Recsim mode
  • Real device, if available
• Supplementary IOCs (..._0n where n>1) run correctly
• Log files do not report undefined macros (serach for macLib: macro to find instances of macLib: macro [macro name] is undefined...

Final steps

• Update the IOC submodule in the main EPICS repo. See Git workflow page for details.
• Reviewer has merged the associated PR for the release notes