Garbage collection for functionClass objects

[abensonca]

Currently reference counting is used for functionClass object memory management. But this fails since we can have reference loops. We currently work around this using a complicated system of recursiveSelf pointers. A garbage collecting approach, perhaps added on top of the reference counting approach, would fix this.

Garbage collection could be triggered manually, e.g. on each MCMC iteration.

Need to keep a list of all allocated functionClass objects. On garbage collection, walk entire tree of objects starting from root(s), marking them as found. Any not marked can then be deallocated.

[abensonca]

A better option might be to allow "weak referenced" in parts of a circular reference which do not prevent garbage collection.