add "milliseconds since epoch" as xUnit for temporal profiles

[Andreas Janz]

Usecase: GEE images have the aquisition date stored in "milliseconds since epoch" in the “system:time_start/end” property.

When sampling and visualizing GEE temporal profiles, it would be nice to have that.

[Andreas Janz]

I just did the conversion to decimal years before adding the profile. Maybe implement this later.

dates = [QDateTime(QDate(1970, 1, 1)).addMSecs(msec).date() for msec in msecs]
dyears = [float(date.year()) + date.dayOfYear() / date.daysInYear() for date in dates]

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