If some internal attributes are saved in the templates, their description
should be checked just like the input attributes to ensure there are no
conflicts.
For every node, the only information that is kept when saving a
template are the non-default input attributes' values as well as the
node's position.
The unit test checking the templates' nodes is updated to stop taking
outputs into account.
This test checks that there is no compatibility issue with the nodes
saved in the template files. It fails when an upgrade of a templates is
needed. Any template can still be opened even if its nodes are not up-to-
date, as they will be automatically upgraded.
