Update molecule forming model to use LJ, generalize for model #2

The hand-written calculation has been removed to use the model's
own LJ forces, which approximates the bond forces and energy
when scaled correctly.

Now that we're using real forces, the large repulsive force you can
obtain so dwarfs the attractive force that it only really makes sense
to make the "Repulsive" text large, to ovoid equating the two forces.

The one issue with this model is that, now that we're using real forces
to calculate a real PE, the model prevents you from dropping the atom
when the PE is positive (i.e. when they are close). We could make this
check optional in MD2D, perhaps.

[#74687984][#74688264]
Create Energy of Bond Formation interactive

This uses the now-generalized model from Forming a Molecule,
and uses the bond-length and energy values from
http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html

The space-fill views are sized such that bonding happens when
the views are overlapping by about 1/3, although this varies
for different combinations (O--O, with a long-range bond compared
to its radius, bonds slightly separated, which also nicely
indicates a weak bond).

One remaining question is what to do with the graph. There is
a reset button, allowing you to clear it, but I'm unsure if we
need to have six different colors representing all six
interactions. (Or if we need to clear the graph for every new pair,
but not doing that is nice because it allows direct comparisons.)


[#74688264]