Matlab getPlane efficiency

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12557

As stated in the ticket linkef above, the original implementation of `getPlane` is inefficient for downloading many planes. This Pull Request adds an extra signature to the method allowing and advanced use where:
- the user can create and pass an initialized raw pixel store to `getPlane()`
- the plane corresponding to the input coordinates is retrieved 
- the raw pixels store is kept active for re-usage
In this situation, it is the user responsibility to 1- close the active service.

A corresponding example is added to the training example (to be tested in the CI job) with time measurements showing efficiency comparison.
NB: if this new signature makes sense, I will extend its implementation to `getTile/getStack` using the same mechanism.

/cc @jburel @mporter-gre