Improve Metagraph interface in terms of drand commitments

[basfroman]

Requested by Jake

meta = bt.metagraph()

meta.commitments
[
 ( block, value) ,
 ( block, value) 
] 

meta.commitments
[
 ( block, decrypted_value) ,
 ( block, value) 
]

sub.get_commitment(netuid, uid, block )
(block, data, False) 

sub.get_commitment(netuid, uid, block)
(block, decrypted_data, True) 

sub.get_all_commitments(netuid, block )
[
 ( block, decrypted_value) ,
 ( block, value) 
]

[basfroman]

Fix bug #2787

[basfroman]

It was decided not to combine the classic and drand based methods of commitments, so as not to complicate the logic and not to confuse users.
Keeping the methods separate is a more transparent and explicit approach.