XLALSimBurstCherenkovRadiation(): new version

- new parameterization of waveform:  source length and radiated flux density instead of natural frequency, beta, and radiated flux density
- add new function XLALSimBurstCherenkov_dE_dA() to compute dE/dA from power, beta, and impact parameter
58 jobs for new_cherenkov_burst in 57 minutes and 58 seconds (queued for 17 seconds)