More explicit steps description and logging

Example :

$  python ~/src/iota2/scripts/common/iota2.py -config configIOTA2.cfg -starting_step 8 -ending_step 8
DEBUG
Full processing include the following steps (checked steps will be run):
Group init:
         [ ] Step 1: create directories
         [ ] Step 2: generate common masks
         [ ] Step 3: compute validity mask by tile
Group sampling:
         [ ] Step 4: generate envelopes
         [ ] Step 5: split region shape by tiles
         [ ] Step 6: extract ground turth by regions and by tiles
         [ ] Step 7: split learning/validation polygons
         [x] Step 8: generate samples
         [ ] Step 9: merge samples
Group learning:
         [ ] Step 10: learning
Group classification:
         [ ] Step 11: generate classification commands
         [ ] Step 12: generate classifications
Group mosaic:
         [ ] Step 13: classfication shaping
Group validation:
         [ ] Step 14: confusion matrix command generation
         [ ] Step 15: generate confusion matrix
         [ ] Step 16: confusion matrix fusion
         [ ] Step 17: report generation
         [ ] Step 18: compute output statistics
         [ ] Step 19: merge statistics


Running step 8: generate samples (5 tasks)
2016-01-17 00:00:00