added clearing and adding operations at a specified index

[JuanRdBO]

Summary of changes

• Added two methods: addOperationAtIndex and clearOperationAtIndex in the TransactionBuilder class

Use Case

I wanted to add them because I was trying to change the amount of one operations after cloning and realized it would that much easier if I could just clear an operation at a specified index and then add it add the updated operation at the same index again.

[Shaptic]

I love this idea! Thank you for the PR. Could you add some unit tests that demonstrate that they function? You can reference /test/unit/transaction_builder_test.js for existing cases and run them with yarn test.

[Shaptic]

Can you run (and commit) yarn fmt? Linter is complaining.