a simple bug to fix

[alundilong]

Dear developers,

I think there is a simple bug on save data in lammps format.
I found the number of impropers are not stored, all the rest are perfect.

Here is an example to illustrate the bug

-------------------------example of the output-------------------------------
LAMMPS 2005 Description - Generated by XenoView

22 atoms
21 bonds
44 angles
58 dihedrals
36 impropers <this part is missing>

2 atom types
3 bond types
................
....

Hopefully, it will help.

Best.,