physics: increase default time threshold for decays

[ManuelHu]

as it was before geant4 11.2...

This avoids confusion and unforseen breakage. Also the higher threshold makes sense for most of our simulations?

what do you think @gipert @tdixon97 ?

see also #135 #158

[tdixon97]

I think its a good idea but we should probably work out what time difference can be resolvable by double with the necessarily precision (idk 1 us). I think this is probably around 30,000 yrs (ie roughly 17 digits for double).
Then we should give an error or warning if the user requests a simulation with an intermediate step having dt > this.

[ManuelHu]

the old default was actually 1e27 ns (~twice the age of the universe), and not 1e60 years.