Allow array index in PV name

[georgweiss]

Pointed out by user: for channel access you may specify pvname.VAL[index] and get the scalar value of the element at index. This PR adds support for the same notation in pv access.

Checklist

• Testing:

  • The feature has automated tests
  • Tests were run

• Documentation:

  • The feature is documented
  • The documentation is up to date
  • Release notes:
    • Added an entry if the change is breaking or significant
    • Added an entry when adding a new feature