Update detectors to check if the other transactions in the group check the transaction using absolute or relative indexes

[S3v3ru5]

The first commit adds helper functions to retrieve the index and field a transaction access instruction pushes on to the stack. The indexes are divided into three categories:

1. Self - Txn.field()
2. Absolute - Gtxn[Int(x)].field()
3. Relative - Gtxn[Txn.group_index() -/+ offset].field()

Given an instruction, the helper function returns the index of the transaction this instruction is accessing and the field it accesses.

The second commit updates the transaction field analysis to track the values of the transaction at an absolute index and at a relative index from the contract. The detectors use this information to consider the validations performed by other transactions in the group. The detectors would not report if an application in the group validates the rekeyto of the logic-sig in a different transaction.

The tests/group_transactions/basic/logicsig_1.py lists different patterns to access the fields. The detectors now identify all such patterns.

[S3v3ru5]

Builds on #189