Add information about block placement to Task object when parsing tapes.

[MarcCote]

This PR adds a new member variable to Task objects, block_changes. This variable provide the block change (in the right order) to reproduce the target.grid structure from the starting_grid.

I added a test file to make sure the data (starting_grid, target_grid, and last_instruction) are the same in the new dataset version (rc3).

Some changes were needed in the code for parsing the datasets.

• [x] Multiturn dataset
• [ ] Singleturn dataset

Todos:

• [ ] Upload the parsed version to mturkstorage.