Improve event logging

[Francesco Casella]

the -lv LOG_ZEROCROSSINGS flag produces output of dubious usefulness, it should probably be removed.

Furthermor, LOG_EVENTS should output the following information

• time of the event occurrence
• type of event (time or state)
• the condition triggering the event

LOG_EVENTS_V should also report this additional information

• for zero-crossing triggered events, the value of the crossing function residual, and, if possible, the number of iterations of the zero-crossing root finder
• any discrete variable that changes its value
• the beginning of a new event iteration