Improved XLS[X] handling
Improved XLS[X] handling

- Issue a warning when xlrd is not available but an xls[x] file has been
  provided.
- Fallback trying to read the xls[x] file with read_csv to handle disguised
  files correctly (yes, this happens quite often in windows-land to simply use
  excel as a viewer).

wavexx/gtabview,scls19fr/gtabview