Add `hash` argument to `Transaction.from_tryte_string`

[todofixthis]

The testnet IRI provides an --export setting which enables users to download raw trytes along with the corresponding transaction hashes.

When loading these values into PyOTA using Transaction.from_tryte_string, it is not necessary for PyOTA to recompute the transaction hash, since the user will be able to provide the corresponding hash.

Modify the signature of Transaction.from_tryte_string to include an optional hash argument:

• If null, compute the transaction hash like normal.
• If not null, use that as the transaction hash and skip curl.

[todofixthis]

Scheduled for release: v1.2.0