First draft on Hydrogen molecule ion exercises, the contain solutions

[bjorgve]

Not ready to be merged. I will add more exercises asap.

[ilfreddy]

1. First eq: s_a \pm s_b
2. Not clear to me what you mean by "neutral function". I would remove that.
3. I assume you are going to remove the cells with the solutions once you are done
4. It would be good to make them plot the different probability densities:
   s_a2, s_b2, psi_p2, psi_m2, 1/2(s_a2+s_b2).
   The last one is to check the difference between having the electron in one of the psi against it being in a hypotetical orbital with the average density of the two original ones.
   In particular, it is instructive to plot psi_p2 - 1/2(s_a + s_b2) and psi_m2 - 1/2(s_a** + s_b**2), which will show where the enhanced electron density for psi_p and psi_m is.

Once they have defined all the functions, step nr.4 is easy.