Add adiabatic pulses to the RF definitions

Based on the sigpy implementation https://sigpy.readthedocs.io/en/latest/mri_rf.html -> adiabatic, I suggest adding a subclass RF->AdiabaticRF with the following classmethod-constructors:

• BIR4
• hypsec
• wurst
• goia_wurst
• bloch_siegert_fm

As it is the nature of adiabatic pulses, the base RF definition needs to be modified to support raster-based frequency modulation, in addition to the phase/frequency offset that is already implemented.