Wrong data type of orbital_parameters in FY4A AGRI reader

[zxdawn]

Describe the bug
The elements of orbital_parameters should be number instead of array.
Otherwise, it can cause the error when it's saved to netcdf file:

  File "/yin_raid/xin/miniconda3/envs/tobac/lib/python3.7/site-packages/satpy/writers/cf_writer.py", line 318, in default
    return [self.default(item) for item in obj]
TypeError: iteration over a 0-d array

Expected behavior

orbital_parameters: {'satellite_nominal_latitude': 0.0, 'satellite_nominal_longitude': 104.69999694824219, 'satellite_nominal_altitude': 35786000.0}

Actual results

orbital_parameters: {'satellite_nominal_latitude': array(0., dtype=float32), 'satellite_nominal_longitude': array(104.7, dtype=float32), 'satellite_nominal_altitude': array(35786000., dtype=float32)}