Improve command arguments of SOM layout

[BerndDoser]

The SOM layout will be defined with --layout, --som-width, --som-height, and --som-depth. To be prepared for future more complex layouts, e.g. growing self-organizing maps, the definition of the sizes should be part of the layout option, so that the new definition would be:

--layout cartesian 5 5 for a 2-dimensional quadratic map --layout cartesian 10 10 10 for a 3-dimensional quadratic map --layout hexagonal 11 11 for a hexagonal map