rescue RT for mzML files which have no scan time, but provide elution time (appararently not the same thing).
And since the default rt is -1 in MSSpectrum this basically checks if it was set before.

Three solutions:
 - use numeric_limits::max()*-1 as new RT default (cannot be occupied by negative RT prediction), but might have other side effects
- introduce new member in parser to indicate that RT was set (and remove == -1 check)
- delete this work-around