Detect cyclic dependencies for calculations

[MartijnR]

E.g.:

• node a has calculation b + 1
• node b has calculation a + 1

Or:

• node a has calculation b + 1
• node b has calculation c + 1
• node c has calculation a + 1

Suggested implementation:

1. iterate through all calculation expressions to build a map of all calculation nodes with the nodes they reference (using xform._extractNodeReferences).
2. then, for each calculation, do a recursive check of all dependent nodes (and their dependencies etc)
3. check if any of the dependencies in 2 is the node itself