EpochsSegmentation should be indexable to retrieve the segmentation per condition

[mscheltienne]

Similar to an epoch object where you can do epochs["my condition"], an EpochsSegmentation should be indexable.
Suggested by Vincent today ;)

c.f. https://github.com/mne-tools/mne-python/blob/6733cae2a0765da9ec1b67a98937839d4cd9aadf/mne/utils/mixin.py#L78-L144