auto grid

[jbernhard]

trento should automatically choose a reasonable grid size based on the collision system. Right now the default is 10 fm regardless of the system, which is ridiculously large for pp and pA. It's very easy to forget to reduce the grid size when switching systems.

trento classes already have enough information to do this, so it should be easy.

The --grid-max option would of course remain as a user override.