add support to initialise NTEnum like NTScalar

[m2es3h]

When initialising an NTEnum with a python dict, it is assumed that the dict is meant for the Value.value and contains index and choices keys for the value. This PR adds ability to initialise value and any other fields of the NTEnum, similar to initialising an NTScalar.

pv = SharedPV(
    nt=NTEnum(display=True),
    initial={
        'value.index': 0,
        'value.choices': ['STOP', 'START', 'STANDBY'],
        'display.description': "Pump on/off control word."
    }
)

[mdavidsaver]

Some coverage would need to be added to TestEnum in src/p4p/test/test_nt.py.

[m2es3h]

This existing test fails because it goes through the isinstance(value, dict) branch in NTEnum.wrap(), which now expects 'value.index' and/or 'value.choices' like NTScalar.wrap() does. Should I update this test, or should NTEnum.wrap() be able to handle a dictionary that only contains fields meant for the Value.value?