Add probe part number as an optional field

[bparks13]

First referenced here, this repo should implement an optional field in the ProbeAnnotations class that can contain the probe part number as a string.

This should expose a public get / protected set, since the probe part number is not something that is intrinsically tied to the ProbeGroup object, so it does not need to part of the constructor. Since it is a string, it can be null or empty to denote that no part number exists for the current object.