patch Model.__init__ (taxa instantiation for mc runs)

[zugnachpankow]

For repeated copan:CORE runs (monte-carlo, pymofa) at the 2nd run following error arises:

Process taxa can only be instantiated once.

This can be solved by using the built-in .delete() method for the entities and taxa. Would be more elegant to solve it with deleting the imported module, del M, where M is the copan:CORE Model. This does not work at the moment.