Passing dt explicitly to readers has a different default behaviour

[IAlibay]

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Expected behavior

For readers which can derive their own dt values, passing a value for dt to the reader should yield the same behaviour across all readers.

Actual behavior

Most readers default to the dt value passed, but the XDR derived readers (XTC, TRR) default to the dt value derived from dt = xdr_frame.time - frame.time and overwrite the dt value passed by in.

[orbeckst]

I am calling this behavior a bug because users can reasonably expect that when they set a value then it is honored.

Please say something if you disagree of the designation "defect". (If it is a defect, we can just fix it for the next release. If it's not a defect then it's an API change and has to wait until 3.0.)