Fix some typos in a notebook
Update README.md
README: Add link to FRETBursts
Fix diffusion coefficient vs sigma relation.

The computed sigma was the standard deviation for the 3D displacement
but it was wrongly used to generate the 1D displacements. This resulted in
simulating an actual diffusion coefficient larger than the value set as input.
The simulate diffusion coefficient was sqrt(3) (~1.7) times larger than it should.
Add notebooks on the theory of Brownian motion simulation
Merge branch 'master' into out_of_core_dev