Provide an add_alias() method to ease alias creation

Currently a record, inside a `RecordWrapper`, can have aliases added to it by calling `add_alias()`. However this is a bit awkward as you have to provide not just the alias'd name but also the device prefix. This often means an application wanting to create aliases has to keep the record prefix accessible everywhere. This is obviously somewhat awkward and could be streamlined by having this logic handled for us in PythonSoftIOC.