Adding a parabolic function in beam generation and some documentation

List of changes

• Added the parabolic bunch generation function, adapted to be same model as bigaussian
• The _get_dE_from_dt was refactored from bigaussian to be used in both bigaussian and parabolic functions. The refactoring was tested see images below before/after.
• Added example to the Ring object to have a momentum program in time

Before (Gaussian)

After(Gaussian)

After (Parabolic)