FCI composites not loadable

[gerritholl]

Describe the bug

The MTG FCI reader cannot load any composites.

To Reproduce

from satpy.scene import Scene
from glob import glob

srcdir = ("/media/nas/x21308/2019_05_MTG_Testdata/MTG"
          "/FCI/FDHSI/uncompressed/20170410_RC70")

fciscene = Scene(sensor="fci",
                 filenames=glob(srcdir + "/*BODY*_006[789].nc"),
                 reader=['fci_l1c_fdhsi'])

fciscene.load(["natural_color"])

Expected behavior
I expect it to load the natural_color composite.

Actual results

Traceback (most recent call last):
  File "mwe14.py", line 11, in <module>
    fciscene.load(["natural_color"])
  File "/media/nas/x21324/miniconda3/envs/py37d/lib/python3.7/site-packages/satpy/scene.py", line 965, in load
    raise KeyError("Unknown datasets: {}".format(unknown_str))
KeyError: 'Unknown datasets: natural_color'

Environment Info:

• OS: Suse 15
• Satpy Version: 0.16
• PyResample Version: 1.11.2

Additional context
I forgot to add the fci.yaml file earlier.