Add threshold for cycles solving by substitution

[mscuttari]

The cycles solving by substitution is a best-effort pass whose application should be limited to reasonable scenarios. At the moment, the only limit is on the number of iterations performed on an SCC to remove the cyclic dependencies. However, a limit on the number of equations of each SCC would also be useful to reduce the computational cost. Such limit should be controllable by the user through an appropriate command-line argument, and default to a low value (e.g., 10) so to keep the optimization active for typically trivial SCCs.

[mscuttari]

Keeping @casella in the loop