Add pulse and gradient fields as StreamItem of mrd format

[kekeke21]

Related to #20 #21
This schema suggestion is to record the pulse sequence and gradients in addition to the existing k-space trajectory field at the mrd_accquisition.yml field. While the trajectory field is kept for backward compatibility with the ISMRMRD format, the additional pulse and gradient fields can also be used to represent the measured gradients and pulses of the scanner.

The phase in mrd_pulse.yml is the time course of the entire scan, exported from crystall oscillator phase data of the IDEA simulator for Siemens, which is not the real-time data from the scanner. At PR #35, adding phase field to mrd_acquisition.yml was suggested which partitions the phase array from pulse field for each corresponding acquisition. This design does allow inconsistent data between the phase saved in the Pulse field and the Acquisition field.

We hope to review this with rounds of feedback sessions on the validity of this suggestion for the ISMRM community and also the availability of such data from Siemens and other scanner vendors.

[kekeke21]

Feedback from MRD meeting:
Rather than creating our own format for pulse sequences, consider representing the pulseq format in Yardl. For streaming, chunk the pulseq .seq file into streamable bytes while keeping track of timestamp coordination.

[kekeke21]

Feedback from MRD meeting:
Rather than creating our own format for pulse sequences, consider representing the pulseq format in Yardl. For streaming, chunk the pulseq .seq file into streamable bytes while keeping track of timestamp coordination.