mavgen_python: make enumerations MavEnum instances

[peterbarker]

... instead of just bare dictionaries

... and add an is_bitmask method to them

# MAV_GENERATOR_STATUS_FLAG
enums["MAV_GENERATOR_STATUS_FLAG"] = MavEnum(bitmask=1)
MAV_GENERATOR_STATUS_FLAG_OFF = 1
enums["MAV_GENERATOR_STATUS_FLAG"][1] = EnumEntry("MAV_GENERATOR_STATUS_FLAG_OFF", """Generator is off.""")
MAV_GENERATOR_STATUS_FLAG_READY = 2
enums["MAV_GENERATOR_STATUS_FLAG"][2] = EnumEntry("MAV_GENERATOR_STATUS_FLAG_READY", """Generator is ready to start generating power.""")

... and one which is not a bitmask:

# MAV_VTOL_STATE
enums["MAV_VTOL_STATE"] = MavEnum()
MAV_VTOL_STATE_UNDEFINED = 0
enums["MAV_VTOL_STATE"][0] = EnumEntry("MAV_VTOL_STATE_UNDEFINED", """MAV is not configured as VTOL""")
MAV_VTOL_STATE_TRANSITION_TO_FW = 1
enums["MAV_VTOL_STATE"][1] = EnumEntry("MAV_VTOL_STATE_TRANSITION_TO_FW", """VTOL is in transition from multicopter to fixed-wing""")
MAV_VTOL_STATE_TRANSITION_TO_MC = 2
enums["MAV_VTOL_STATE"][2] = EnumEntry("MAV_VTOL_STATE_TRANSITION_TO_MC", """VTOL is in transition from fixed-wing to multicopter""")