Improve precision of timings
This issue is twofold:
- We should increase the precision of timings, moving from millisecond to microsecond, to obtain improved stats in
why3 session info
(see also #762 (closed) ) - Alt-Ergo sometimes reports taking zero time. Also, it is the only prover for which the reported time is parsed instead of asking the server. This should be uniformed, possibly by using server's times for Alt-Ergo.