Add Synchrotron Radiation model

[Mateasek]

I would like to add a Synchrotron Radiation (SR) emission model to Cherab. The radiation will be based on the local properties of the electron distribution function. Because of the directivity of the radiation, I would like to provide users with a number of integrators suiting well wider range of speed and accuracy requirements. For this reason I will implement 2D integrators (raised in a separate issue).

TODO:

• [ ] radiated spectrum
• [ ] radiated power by a single electron
• [ ] directivity of the radiation
• [ ] 2D integration over polar coordinates in radiator's frame
• [ ] tests
• [ ] documentation