Inconsistent naming of optional datasets in composite configs and compositors

[pnuu]

Code Sample, a minimal, complete, and verifiable piece of code

# visir.yaml
  nir_reflectance:
    compositor: !!python/name:satpy.composites.NIRReflectance
    prerequisites:
    - 11
    optional_prerequisites:
    - solar_zenith_angle
    - 13.4

# composites/__init__.py
class NIRReflectance(CompositeBase):

    def __call__(self, projectables, optional_datasets=None, **info):
        pass

Problem description

For optional datasets the naming is different in the YAML configuration files for the compositors compared to the variables used in the compositors.

Versions of Python, package at hand and relevant dependencies

All SatPy versions.

[pnuu]

My preference would be optional_datasets in both, I believe that would require less work and would be easier to write ;-)

[djhoese]

I think we need to standardize all of this. prerequisites versus datasets versus projectables. projectables is our old naming before we started calling things datasets, so I think that can go. If the YAML uses datasets that gets a little confusing since it doesn't have the same meaning as prerequisite or maybe dependency.

[djhoese]

Closing this as a duplicate or at least part of #161. We will standardize these names when that issue is resolved.