SciDAVis - Improvements SVN-temp

between double, QString and QDateTime any more. This means you have to
cast AbstractDataSource to its subclasses before you can read the actual
data.
- Added conversion filters DateTime2Double, DateTime2String, Double2DateTime
and String2DateTime based on the prior code for implicit conversion.
Filters for month and day based conversion yet to do. Need to investigate
two failing unit tests.
- Added automatic type conversion to ReadOnlyTableModel.
- Fixed some bits in TableModel.
- Filter test program demonstrates a basically working edit->filter->display
chain; some things still need to be fixed, though.
- Some small improvements to filters and their documentation.