Refactor

A little refactoring of spat plugins. In the process I added two new fields to the plugin definition files: numChannels and description. The configuration now takes the spat name (directly) and numChannels to configure number of supernova channels. In the end, we use spat names so we don't need to maintain the long case statement when we add or remove spatializers. We can now query all plugins for their names and descriptions (SatiePlugins.showPlugins, from sclang only, for now) .