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.

[kurup]

Just wondering if this has been resolved yet? Let me know if there is anything I can do to help. Two issues (this one and #153) can then be closed for the price of one!

[m2es3h]

@kurup I'm happy with this PR as it is now, just needs review and approval on your end.