Rename RecordingChannelGroup to ChannelIndex

[apdavison]

This follows the discussion in #210. Since individual RecordingChannel objects have been removed from the proposed Neo 0.4 API, the name RecordingChannelGroup no longer seems appropriate.

[samuelgarcia]

Hi andrew.

OK for this.

The only litlle comment I could do is that we lost here the semantic of Group.
Maybe this not important.

A typical use case even if noby use I guess is:
We have 128 channels that come from 2 diffrents arrays of 64 electrodes in the same recording.

signals = neo.AnalogSignal(np.zeros(nb_sample, 128), units='uV', sampling_rate=10*pq.kHz)
indexes_A = neo.ChannelIndex(name='shank A', channel_indexes=np.arange(0,64))
indexes_B = neo.ChannelIndex(name='shank B', channel_indexes=np.arange(64,128))

Maybe the semantic of ChannelGroup is easier to catch. I do known.

This is just to have your opinion on that I don't care so much.
So OK for me for merge.

Thank again for all the work in apibreak.

[coveralls]

Coverage decreased (-0.02%) to 57.452% when pulling 3dcc9da on apdavison:channelindex into 066777b on NeuralEnsemble:apibreak.

[coveralls]

Coverage decreased (-0.04%) to 57.431% when pulling b4e7be6 on apdavison:channelindex into 066777b on NeuralEnsemble:apibreak.