Lennard-Jones parameter definitions in interaction_functions.py

**Description of the error**
While trying to add more force field for more complex sugar-molecules I discovered a problem.
We don't have clear definitions for Lennard-Jones potential values. In the code interaction_functions.py we have defined values or parameter set-ups for charges, angles, dihedrals, which can distinguish more or less between parameters for different atoms, but this option doesn't appear clear for Lennard-Jones potential. I general it looks like we can have only one value for the whole simulation which we can respectively give for one atom.

**Describe the expected result**
We supposed to have options for different values for different atom-pairs

**Describe the actual result**
In the actual result we input only one value.

**Suggested fix**
Make it for multiple values and well-distinguishable.

**Additional details**
The piece of code is here on the image