Improve sidemax default behavior in packing.py #1257
Description
Describe the behavior you would like added to mBuild
With PACKMOL 21, there were some bug fixes that impacted the speed and memory usage (#1256). I noticed the memory usage and time performance are especially sensitive to the sidemax parameter. Right now we use a default of 100nm. There should be no reason to ever use a sidemax value larger than the volume you're packing.
Describe the solution you'd like
If sidemax isn't specified by the user, the default behavior should be to calculate it from the box used for the packing method.