Clean-up of TrackRecord header ahead of further updates

Cleaning up the TrackRecord header in preparation to migrate it from being based on truth barcodes to be-based on truth status and id.

Most changes relate to renaming GetBarCode() to the more standard barcode() and SetBarCode() to SetBarcode().

I may make a follow-up MR further standardising method names, but I prefer to get the current changes in first.