Improve documentation of interfaces related to R-wave and ECG data

[jogerh]

Image3dAPI/IImage3d.idl contains interfaces for transferring R-wave and ECG information.

EcgSeries could meaningfully be used to transfer only trig points, without ECG trace. DICOM files typically only contain the R-wave times, not the trace values. EcgSeries samples could also benefit from a document update to highlight this.

In addition, it may be cases where a loader returns trig points where some intervals contain no frames. This could be the case for multi-cycle acquisitions, where the full volume is composed of data acquired over multiple cycles. It would then be expected behavior for a loader to return trig intervals without frames. A client that displays QRS/trig points and/or trace could then benefit from trimming the displayed data. The documentation could be updated to highlight this.