Sorting issues in BoltArraySpark

[boazmohar]

As a follow up on the discussion with @freeman-lab in thunder-project/thunder#325
I suggest the following changes:

1. Renaming _ordered to _sorted
2. Adding a sorted property returning _sorted
3. Adding a sort() function that would check _sorted and sortByKey the rdd if needed
   @jwittenbach what do you think?
   If you approve I will try to implement.