Add ArbitraryGradient from k-space trajectory

Based on the publication and implementation of M. Lustig, sigpy offers this function already:

https://sigpy.readthedocs.io/en/latest/generated/sigpy.mri.rf.min_time_gradient.html#sigpy.mri.rf.min_time_gradient

As sigpy is added as dependency for RF pulses anyway, it might be handy to wrap this as classmethod of the ArbitraryGradient class