A number of things weren't right with this, likely for a while, since
we had no tests or examples in the tree. This adds three user-supplied
sample files and a test to hopefully keep it working. Fixes:
- Ignore blank entries
- Tolerate a missing "Channel Number" column
- Support the "Scan" (meaning not skip) column type
- Fix detection by being looser about what we look for
Fixes#11389
