Improve plugins
For now all name requested return one value per frame. There is already internally some mecanism to return a scalar or an array according to the value. We should had this to the 'instrument_keys' and 'positioners_keys' provided by the user to return a scalar or an array.
This can be tricky for users. I guess we could expect something like:
instrument_keys = (
("hry_delta/data", "array"),
("fscan_parameters/y_npoints_arr", scalar),
("hry_trig/data", "array"),
)
Default value would be an array I guess