Add functionality to RFPulse to compute bandwidth and rf_centers from the RF waveform

Right now it is expected, that the sbuclass of RFpulse calculates the bandwidth and rf-centers.

To not break compatibility, we could also make these optional arguments to the RFPulse constructor, and if not specified a function is called along the lines of these two:

https://github.com/imr-framework/pypulseq/blob/dev/pypulseq/calc_rf_bandwidth.py
https://github.com/imr-framework/pypulseq/blob/dev/pypulseq/calc_rf_center.py

Furthermore, the waveform must be real-valued to allow the functions above to work. Right now this is not enforced. There are cases where complex waveform defintions are directly handed to the constructor, but I am not sure right now why.