Create a new API for generated clasess

[roigcarlo]

We should discuss which are going to be the modules and where to put the files for the generated classes API.

[kitchoi]

We now have the generated classes and an api.py in simphony/cuds/meta

[mehdisadeghi]

@kitchoi Since this issue was about where to put classes, I will close this one and open another issue for the defining a common functionality API of the meta classes.

fixed in #300