ENH?: add write epochs function

[dengemann]

I thought we had it already. ...

cc @agramfort

[agramfort]

epochs.save ? why do you need the write_epochs ? we have it for evoked to write multiple evoked in one file.

[dengemann]

For fieldtrip to make epochs writing easier / happen? AFAIK it is not yet supported. People repeated me ask me how to export their epochs from matlab. Was thinking also about this one.

[agramfort]

I read that you want readers for fieldtrip epochs .mat files. Why not if there format is quite stable in time.