MRX: Pulseq

TODO

A rudimentary pulseq API is currently included in [toolapi]. It should be extracted into its own package by copying pulseq-zero. This package should be toolapi - compatible while being able to write .seq files without having dependencies outside of MRX.

Contrary to pulseq-zero, MRX: pulseq will initially not be differentiable until support has landed in the toolapi.