Retrieve once and store the enum labels of the DevEnum attributes

[kmadisa]

Suggested in PR#46: At the moment we instantiate a device proxy and fetch the enum labels every time we check if the command should be allowed to execute. We need to find a way to do this just once and store them for future use while the device is running. (hint: to this in the always_executed_hook method).

[ajoubertza]

There were some recent improvements to enumerated types in PyTango, so this can be done better.

Example of the type of code we want to fix:

    def _is_command_allowed(self, command_name):
        dp = DeviceProxy(self.get_name())

        obstate_labels = list(dp.attribute_query('obsState').enum_labels)
        obs_idle = obstate_labels.index('IDLE')
        obs_ready = obstate_labels.index('READY')
        ...
        if self.read_obsState() == obs_idle:
            ...

Attributes are currently instantiated like this:

    obsState = attribute(
        dtype='DevEnum',
        doc="Observing State",
        enum_labels=["IDLE", "CONFIGURING", "READY", "SCANNING", "PAUSED", "ABORTED", "FAULT", ],
    )

But now that can be done directly from an enumerated type, so rather:

class ObsState(IntEnum):
    IDLE = 0
    CONFIGURING = 1
    READY =2
    SCANNING = 3
    PAUSED = 4
    ABORTED = 5
    FAULT = 6

class SKAObsDevice(SKABaseDevice):
    obsState = attribute(dtype=ObsState)
    ...

That allows the initial code to be simply:

    def _is_command_allowed(self, command_name):
        ...
        if self.read_obsState() == ObsState.IDLE:
            ...

However, this is another area we some changes are needed in POGO. The high level code generation should create enumerated types in this new way. There is a short-hand form of creating enumerated types that may be more convenient for POGO:

ObsState = enum.IntEnum('ObsState', ["IDLE", "CONFIGURING", "READY", "SCANNING", "PAUSED", "ABORTED", "FAULT", ], start=0)