When the nodes.VariableRefAttributeAccess and nodes.VariableRefMethodCall were added, the
node comparison function were broken since the class instances are not the same.
This is a problem when using the groupby and an attribute access or method call on projection
variable.
For example:
Any X, S.download_url() GROUPBY X,S WHERE X is ImportProcess, X shacl_report S

were a bad rql query since `S.download_url()` is not the same node (not the same class) as `S`.
But actually it is.

This patch allow this syntax