Make `mne.io.Raw` available as `mne.Raw`

Two of the most important data classes, mne.Epochs and mne.Evoked, are available directly in mne. However, the class for continuous data is available as mne.io.Raw and not mne.Raw (because all read_raw_* readers also live in mne.io). For consistency, I propose to make mne.io.Raw available as mne.Raw (but keep mne.io.Raw for backward compatibility). WDYT?

[drammock]

@cbrnr’s argument makes sense to me too, but I agree with @agramfort that the root namespace is over-crowded. I would not do this as a one-off change, but I would consider doing this as part of a larger namespace overhaul, done after careful consideration/planning, and with a longer-than-normal deprecation cycle.

[cbrnr]

+1 from me because IMO the three most important classes that represent data (Raw, Epochs, Evoked) should live together. @larsoner there is at least one read_raw_* function which produces an Epochs object (read_raw_eeglab()). @agramfort I’d rather move less important things into submodules and have these three classes in the root.