Dihedral in OPLSAA can not have more than 3 parameters defined

**Description of the error**
When creating a new molecule and adding parameters for the dihedral potential according to the manual of MDMC one shall be able to add up 6 additional terms where each one contains of 3 values: K (force constant in energy values), n (multiplicity) and d (phase).
Practically this doesn't look feasible because the software wants only K1, n1 and d1 (only 3 parameters which describe one component out of 6 possible). Basically, if a use wants to add manually values which weren't found in the force field it is not possible to do for more than one compound.

**Describe the expected result**
I expected a successful addition of parameters

**Describe the actual result**
Dihedral potential wasn't added

**Suggested fix**
Check out where the limitation is implemented and change it to up to 6 values.

**Additional details**
As an additional detail I add a screenshot of the failure.