Method find_param_to_subs() commented out

I am not sure why I have written it...
eample how to use the method replace_singe_var()
Method sys_dict_to_list() created
Example how to use sympy.cse

Here a short example how to optimize s set of equations with the sympy's
function cse
Bugfix in sys_dict_to_list()

The keys of the dict should be sorted first and then converted to list of equations
Example how to print the source code of a ODE system