DOC: Return empty EcgSeries if EGC is not available

[forderud]

Proposal to clarify API documentation as requested in #115.

I don't really have any preference on if lack of ECG should be reported as E_FAIL (or other error code) and/or an empty ECG struct, so we can just as well fail if there's a request for that.

[jogerh]

Would an S_FALSE success code make more sense here?

[jogerh]

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.

[forderud]

Would an S_FALSE success code make more sense here?

The challenge with S_FALSE is that languages like C# and Python report it just like S_OK. As far as I know, it's therefore not possible to detect S_FALSE specifically from these languages. See https://blogs.msdn.microsoft.com/jaredpar/2005/10/14/getting-hresults-back-from-com-interop-calls/ for more info.

[jogerh]

Further doc update will be addressed by #139