This prevents ambiguities between `NodePlugin` objects that have been
registered (and are thus instantiable) and those that belong to `Plugin`
objects but have not been registered.
By performing a `initNodes()` every single time `checkTemplateVersions`
was called without ever unregistering the nodes, warnings about nodes
being already registered were raised when for example calling
`checkAllTemplatesVersions`.
For now, it contains methods allowing to check whether a template provided
as an argument is valid (i.e. has no compatibility issue).
It performs a check that's very similar to what `test_templatesVersions`
was doing, but it allows this check to be performed from outside of
Meshroom.
