The solution is to use FunctionWrappers.jl which provides a high
interface for C function pointers.

All tests passed.

implemetend in the package.
A lot of files has been modified because some of the internal syntax has
changed. But the top level methods should still work.

Two tests about the plots of synchronized oscillatory simulations have
been added.

All tests passed.

allocation per next_state!. Now update_state! functions returns a Symbol
as location instead of modifying a vector.

check_constraints and update_state! functions of an Edge in order to
improve performance.
Acutally the gain is low, but it improved the readabilty of the code.
All tests passed.