Adds year to filename sorting

[cassandra-elmer]

When using MultiDBD, files were being read in order:

• '*-10.dbd'
• '*-11.dbd'
• '*-12.dbd'
• '*-0.dbd'
• '*-1.dbd'
• '*-2.dbd'
• '*-3.dbd'
• etc.

This made for problems with MultiDBD.get_sync, as the times weren't monotonically increasing, so interpolation failed and every field returned nan. Adding the year to the regex and sort key fixed the problem.

[smerckel]

@cassandra-elmer: thank you for the patch. In fact I made a test in dbdreader_test.py (test_sorting()) that sucessfully sorted the filenames, also for files with different years. When constructing my test filenames, I accidentally omitted the day of year the mission started, so the test never failed. Thanks for spotting this! I also update the test_sorting() function.