SciDAVis - Improvements SVN-temp

- Fixed compilation issue introduced in r1203.

- Documented class Matrix.

- Fixed repeated error messages when evaluating matrix formulas.

- Documented class Table.

- Reduced default # of decimal digits from 14 to 6 (for new installations
without saved settings). This seems to be a slightly saner default for
most applications, given that high accuracies are seldom reached in practice.

- Documented class MDIWindow.

- Commented out various Python functions which are not usable in the current state.

- Completed documentation of Column class.

- Started to rewrite documentation for Python API. Most of AbstractAspect will
remain undocumented, because it can't be used sensibly without a completely
Aspect-based project management and because some parts will change in
a future release. The same goes for Column::dataType(), which is already
removed in LabPlot SVN. Otherwise, the new "API documentation" section is
supposed to cover everything exported in scidavis.sip, while I'm planning
to shorten the section "Accessing SciDAVis's function from Python" to a
tutorial-style overview over the most important features.

- Fixed documentation of muParser column() function.
- Fixed missing initialization of AssociationsDialog member (just in case).