Move use_data_service from PVOutput._api_query to PVOutput constructor

[JulianBriggs]

Move the attribute use_data_service
from PVOutput._api_query (to which is is passed by for example get_status(...use_data_service=True)
to PVOutput constructor eg:

def __init__(
    self,
    api_key: str=None,
    system_id: str=None,
    config_filename: Optional[str]=CONFIG_FILENAME,
    data_service_url: Optional[str]=None,

use_data_service: bool=False,
):

Detailed Description

Using the data service (rather than free service) is a connection level thing.
Better to set once on creation of PVOutput instance

Context

Counter-intuitive to set in every call to get status etc

see above

[peterdudfield]

Good idea