Don't perform ICA cleaning on raw data by default when epochs are present

[hoechenberger]

Fixes #925

Before merging …

• [x] Changelog has been updated (docs/source/changes.md)

[hoechenberger]

I'm not sure this is the right approach (in terms of, where to do the conditional branching)… @larsoner Could you kindly take a look?

[hoechenberger]

Thanks for your help here, @larsoner, this is much appreciated!

I'd like to hear @SophieHerbst's input as well, esp. since she said they were struggling with disk space usage and all…