Clean up unused imports in csv.py
scope should not be mutable by default
Clean up tests
Remove expressions
Implement drop
Remove create and drop index classes
Add dropping tests
Remove create index drop index from sql compute
Move drop to respective types
Implement drop for pytables
Add test for dropping a pytable
Implement drop for HDF5
Add test for HDF5 dropping
Implement drop for mongodb
Add test for mongodb drop
Implement drop for csv
Implement drop for sql