Configs/IOC Settings: Adapt Precision settings

As a user I would like to set the precision in use for a block (or PV) easily, and not have to re-enter it every time I start the IOC.

== Acceptance Criteria ==
1. The .PREC field is added to autosave
1. There is a way in the configs to specify what the precision value should be

== Notes ==
1. The gui paying attention to the PREC field means that in some situations the wrong precision can be applied (e.g. PREC = 0 on SDTEST, which highlighted this issue, but could be of use in other IOCs)
1. Autosaving the PREC will mean that the changes don't have to be done continuously
1. The configs could be altered so that blocks have a precision value associated which can be written back to the PREC field of the associated PV in code
1. There is a workaround available by adding the PV in question with the ".PREC" suffix to the list in the IOC PV Values