fixes #1196

also, fixes a bug introduced in PR #1189, when not using stabilization

- add memory usage estimator
- test against varying number of atoms
- test against non-consecutive atom tags
- test for 32-bit overflow in number of atoms
- test for 32-bit overflow in timestep
- reduce tail correction error to warning
- more LAMMPS style formatting of the source code
- remove trailing whitespace
- avoid leaking memory from allocated arrays for masses/charges/tags

enables consistent syntax when using custom groups