TODO:
* test the calculation of new fit goodness parameters. Compare with the results of R maybe or gnuplot (http://gnuplot.sourceforge.net/demo_5.0/fit.html).
* add automatic tests for fitting using NIST data e.g. (http://www.itl.nist.gov/div898/strd/nls/nls_main.shtml).
* test 7e7c0e4dd08b4a15d4ce251e64ca267bb2d9e25e on windows.
* use KSharedConfig instead of KConfig?
* Remove the usage of KDialog, swith to QDialog. Following dialogs are affected:
** DatabaseManagerDialog
** ImportDialog
** FITSHeaderEditNewKeywordDialog
** FITSHeaderEditDialog
** FITSHeaderEditAddUnitDialog
** GridDialog
** ExportWorksheetDialog
** DropValuesDialog
** EquidistantValuesDialog
** ExportSpreadsheetDialog
** FunctionValuesDialog
** MatrixFunctionDialog
** HistoryDialog
* consistenly use the DEBUG macro, remove the usage of
```
#ifndef NDEBUG
#include <QDebug>
#endif
```
* restoring of dialog sizes doesn't work. KWindowConfig::saveWindowSize(windowHandle(), conf) and KWindowConfig::restoreWindowSize(windowHandle(), conf) don't seem to do the correct job.