Exporting metamodel

[sbragagnolo]

During the exporting of a metamodel, whatever object in moose properties that do not belongs to the meta model and is not a simple object (string / bool / etc) is ignored without warning nor error.

It may be a good behaviour for some usages, but we should allow the activation of exceptions (or deactivation), for the process of development, to ensure that we can understand what is not being exported, when and why.

It could be also nice to article some secondary serializer for those objects that we cannot export with MSE such as Fuel or Ston. Like this we can keep our "boundary" objects (such as an smacc ast )