Automatic integration for decays

[kilian]

The present implementation requires an explicit integration for all decay channels before a particle can be declared as unstable. WHIZARD should be able to do this integration automatically when the process is requested as a decay. Moreover, 2-body decays always have trivial phase space, so the partial width can be computed and the process object can be set up without any integration at all.

[kilian]

Resolved in r2406, which does even more: integration is automatically called both by unstable (for decays) and simulate (for any process), if the processes are not found in the store.

Omitting the integration altogether is not so relevant anymore, since for 1->2 processes the number of calls is only 1x100. It might be useful for 2->2 processes, if no simulation is intended.