[Walls Import] bugs when `#PREFIX` is changed multiple times in a survey file

[jedwards1211]

Per Joe Walko:

I did find one data ingestion bug: At least in some cases, Breakout is not picking up the second #PREFIX directive when a Wall's SRV file contains more than one prefix. Thus, all the stations in the SRV file get assigned the first prefix, which breaks ties. I'd need to do more testing to see if that happens in all cases when there are multiple prefixes per SRV file.