gmso FF periodic dihedrals and impropers yield a unyt_quantity or unyt_array

[bc118]

gmso FF periodic dihedrals and impropers yield a unyt_quantity or unyt_array.

unyt_quantity : if only 1 periodic value is provided (1 n-value per dihedral/improper). See the gmso_benzene_GAFF_1_n_values.txt for an example.

unyt_array : if only 2 or more periodic value is provided (>2 n-value per dihedral/improper). See the gmso_benzene_GAFF_mulit_n_values.txt for an example.

gmso_benzene_GAFF_1_n_values.txt
gmso_benzene_GAFF_multi_n_value.txt

We may want to move all of them to the unyt_array for the GMSO FF periodic dihedrals and improper because it will make the engine writers less complicated and easier for the users to understand.