Packet Validation - Python 2 to 3 update

In tlm.py, the validate() functions declared in the FieldDefinitions and DerivationDefinition classes need to modified to fix the 'else' statement of following block (found in both methods):

    if self.enum:
        if value not in self.enum.values():
            valid = False
            flds = (self.name, str(value))
            log("%s value '%s' not in allowed enumerated values." % flds)
        else:
            primitive = int(self.enum.keys()[self.enum.values().index(value)])

The index() method is not a valid method on the enum object.

'self.enum' is a dict:
FieldDefinition(_bytes=0, desc='Distinguishes between core and p...', dntoeu=None, enum={0: 'Core', 1: 'Payload'},

That line will have to be rewritten.

The following script will cause the problem:

import ait.core.tlm as tlm
tlm_dict = tlm.getDefaultDict()
ccsds_pkt_def = tlm_dict['CCSDS_HEADER']
data = bytearray(b'\x01\x02\x03\x04\x05\x06\x07')
ccsds_pkt = tlm.Packet(ccsds_pkt_def, data=data)
test = ccsds_pkt.validate(messages='test validate')

Start in _tlm.py with the validate() methods in FieldDefinitions and DerivationDefinition, then reproduce the failure using the Python script in the issue. Update the invalid enum lookup in both methods so packet validation completes, and verify that the supplied CCSDS packet validates without the reported error.