reftran should propagate msr <Source> records when using --export-xml-files

[jhaasdyk-au]

Brief description

1. Reftran does not currently populate 'Source' records (for msrs) when writing xml via --export-xml-files.
   Import on the other hand, will return supplied 'Source' records when writing xml via --export-xml-files.
   This can be demonstrated in v1.2.7, v.1.2.8, but has also been noted in earlier versions.
2. DNA format (for msrs) does not currently support 'Source' records.
3. Compare to the attribute 'Description' (for stations) which is fully supported in XML, DNA, and exports to both.

Basic requirements

1. 'Source' records (for msrs), when provided to import.exe, should be re-exported in all cases of --export-xml-files.
2. Lower priority: DNA format (for msrs) could be extended to include 'Source' records & support via --export-dna-files.

Priority
Priority 2 - Medium