SciDAVis - Improvements SVN-temp

- Implemented more sophisticated conversion from numbers to date/time values.
Julian days are seldom a good choice because QDate only supports
days with JDN greater or equal to 1, and using noon as reference time probably
isn't always what one wants. Additionally, only numbers above 0.5 yield a
valid QDate, which is somewhat counter-intuitive; especially when one uses
times given as fractional days.

- Plotting of date/time columns essentially works, but could do with more
testing. Changing the axis label type to date/time after plotting numeric
columns at least doesn't crash or display error messages any more, but is
considerably less useful than converting the columns before plotting; interpreting
numbers as times, dates or date/times always assumes milliseconds since midnight
or Julian day numbers, respectively. However, simply changing these conventions
would make importing legacy project files a nightmare.

- Empty reference date specifications allowed in some places, meaning Julian day
epoch (January 1, 4713 BC). This is necessary because JD must continue to be
supported for backwards-compatibility, but its reference date
(QDate::fromJulianDay(0)) is outside the range QDate can handle.