Build models with more than one attribute should be done by tensorial sum

[SergeStinckwich]

This is possible at the moment, to create models with two (or more) attributes:

KEModel new
	attributes:
		{(#status -> #(#S #I #R)).
		(#species -> #(#mosquito #reservoir1 #reservoir2))}.

but these kind of model should not be created directly like that but with a tensorial sum of two concerns instead.