OMERO.matlab: add utility function to load channels

This PR was initiated from a discussion with @bramalingam and another user of the toolbox in Edinburgh. Changes included are:
- adds a `loadChannels()` function to retrieve the channels from an image
- extends the `LoadMetadataAdvanced.m` training script to use this method
- extend the `CreateImage.m` training script to use this method and show how to populate channel metadata

To test it:
- check the training examples work
- check the following signatures work and return the loaded Channel objects

 ```
channels = loadChannels(session, image);
channels = loadChannels(session, imageID);
 ```

/cc @imunro