Improve Verlet buffer constraint estimate

Improve Verlet buffer constraint estimate

The displacement estimate for a constrained atom (typically H)
rotating around the COM with a partner atom was not thoroughly
derived and the code documentation was not fully correct.
Now there is an exact MSD integral. Approximating that gives
the same v/(1+av+bv^2) variance scaling formula with a=1/6, but b
changed from 5/180 to 8/180, slightly lowering the buffer size.
Note that we (still) use a Gaussian with matched variance, which
results in a much larger buffer than necessary, since the tail of
the distribution sets the buffer size and the Gaussian has a long
tail whereas the actual distribution has no tail.

Also added a test for this estimate.

Change-Id: Ie6fb817dcd55177e5992facc7b68616f318572a3