Support for recursive functions

Recursive functions are needed to implement iterative algorithms with an exit test. Use case where recursive functions are necessary:

• for cycling coupled coarse grained and Lattice Boltzmann models
• for self-consistency type iterations

Recursive functions can be parsed and checked. However, typing and evaluation are currently not possible / not implemented.

The technical implementation has some commonalities with the short circuiting implementations, see issue #49 (closed).