- script to generate a database to reproduce the bug
- no crash observed with qgis natural breaks (jenks)
- added warning and chance to cancel Natural break classification for
  dataset >50k records
- modified test script to generate gowing sizes tables (for timing)
- improved wraning msg, removed useless test
Use scale parameter to display concerned class
Fix class name
Merge pull request #720 from Oslandia/issue6483

Fix #6483 - Thanks Vincent!
Merge pull request #894 from sbrunner/legend-scale

Use scale parameter to display concerned class