typos + comments on documentation

[georgestod]

Typos
in https://github.com/PhIMaL/DeePyMoD:

• first sentence in : 'noisy' instead of 'noise'.
• first sentence below figure: 'available' instead of 'availeble'
• in https://phimal.github.io/DeePyMoD/api/data/burgers/, the parameter 'v' represents viscosity not velocity
• for the data API: BurgerCos implementation is an implementation of 'almost tangent' initial conditions (cosine for the heat equation). When b=0, the initial conditions are exactly tangent initial conditions.

Comment on doc

• a proper explanation of periodicity should be given and its distinction with the frequency at which stuff is computed for plotting purposes should be made