Gridding a sequence containing trapezoidal gradients results in 'cut off' corners and a time-grid shift of grad_raster_time/2 about pypulseq HOT 4 CLOSED

@jweine Issue #72 was related to this, fixed in PR #84. Please try it out and let me know!

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@sravan953 Negative, unit test still fails:

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FAIL: test_correct_start_trap (GradWaveformUT.TestGradientWaveform)
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Traceback (most recent call last):
  File "/Users/bilal/Insync/[email protected]/GoogleDrive/codebase/pulseq_sequences/bilal_seqs/GradWaveformUT.py", line 16, in test_correct_start_trap
    self.assertTrue(np.allclose(wf[:, 0], 0, rtol=1e-10),
AssertionError: False is not true : First gridded sample is not zero

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I feel like the issue arises from the use of interpolation in the points_to_waveform(), as @jweine pointed out. I will take a look at the issue and see if I can come up with a better solution.

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Update: Behavior is the same between 'Pulseq' and 'PyPulseq'. 'The issue' comes from the convention Siemens uses, shifting gradient sampling by half raster time, or equivalently, sampling the gradients at the center, not the edges. I believe confusion comes from the fact that, we use shifted gradients for visualization, when in fact, it makes more sense to display non shifted ones.

If we also plot the time-axis, as the first sample will not be at time=0, but time=GRT/2, it all makes sense. So I believe there is no bug here, just a confusing feature.

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@bilal-tasdelen Thanks for following up on this. The existence of this shift is mentioned in the pulseq file format definition when using arbitrary gradients and rf-waveforms but not Trapezoidals. But I couldn't find the actual reason for using the shift / center-sampling. I feel it should be documented that this is a Siemens specific feature, as i don't think it is intuitive without more context (especially for non-siemens sites :)). Personally I think, this behaviour should not be presented to user but handled in the Siemens sequence export. But this is more of a design decision to be discussed also with the Pulseq project. Anyway, thanks for the clarification!

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