OMEROSession: make pixels access higher level

There is now an openPixels() method which returns the raw pixels store
to use for reading the pixels. This simplifies OMEROFormat's Reader.
OMEROSession: add createPixels method for writing

This method configures a raw pixels store for writing to the pixels
of a newly created Image, and will be instrumental in implementing
the OMEROFormat's Writer.
OMEROFormat: add first Writer implementation