Fill out min and max

[dpgrote]

This enhances the min and max wrappers, adding keyword arguments with defaults, error checking of the input, and doc strings.

It would be nice to have this level of wrapping for all of the routines, but this would be a fair amount of work. The error checking is fairly important since wrong values can give either just bad results or seg faults.

[ax3l]

Thank you, this is great!

Some of the error checking, e.g., if used outside of hot loops, could also be done on the AMReX C++ side.

A clean throw will propagate to our level as long as we compile with RTTI enabled #76