Why does `reflectance` accept multiple source images and `radiance` does not?

`reflectance` takes a list of sources as input:
https://github.com/mapbox/rio-toa/blob/0a40beb1bd3c05c76c65f2f20951591d831ad5ea/rio_toa/reflectance.py#L131-L137

while `radiance` takes just one and expects that it contains multiple bands:
https://github.com/mapbox/rio-toa/blob/0a40beb1bd3c05c76c65f2f20951591d831ad5ea/rio_toa/radiance.py#L64-L70

Why the difference? It should be easy to get the same behaviour in `radiance` with the principle used in `reflectance`:
https://github.com/mapbox/rio-toa/blob/0a40beb1bd3c05c76c65f2f20951591d831ad5ea/rio_toa/reflectance.py#L93-L96

If there is no real reason for having two different interfaces, I suggest we harmonize them.

Compare the cited input-handling sections in rio_toa/reflectance.py and rio_toa/radiance.py, starting at the referenced lines. Determine whether their different interfaces are required by the calculation, then define and implement a consistent radiance input behavior if not. Done means the interface decision is clear and the relevant behavior is covered by tests, though no test file is named in the issue.

Written by the indexing model from the issue text.