XML numeric file reader

This is a first implementation of a file reader for an xml-numeric file, based on discussions with @endJunction, @TomFischer, @nagelt and NB.
I designed a schema-file and the reader currently only validates files according to that schema, reads the information and outputs everything that has been read (as there is to my information currently no data-holder-object for numeric information).

An example file would look like this:
```xml
<?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type="text/xsl" href="OpenGeoSysNum.xsl"?>

<OGSNonlinearSolverSetup>
  <Type>Newton</Type>
  <LinearSolver Library="PETSc">
    <Type>BiCGStab</Type>
    <Preconditioner>diagonal</Preconditioner>
    <!-- more parameters here -->
  </LinearSolver>
  <IterationScheme>
    <MaxIterations>10</MaxIterations>
    <FixedStepSize>5</FixedStepSize>
    <!-- more parameters here -->
  </IterationScheme>    
  <Convergence>
    <Method>L2Norm</Method>
    <ErrorThreshold>1E-10</ErrorThreshold>
    <!-- more parameters here -->
  </Convergence>
</OGSNonlinearSolverSetup>
```

Note that implicitly every configuration is assumed to be non-linear. According to @nagelt this isn't really an issue as you could always choose ```MaxIterations=1``` for a linear problem.

Please let me know if this is basically okay or what needs changing otherwise. (Aside from various parameters that can be added later...)
I am unsure with the Qt stuff: Is it possible and usefull to write ```QDomElement const& num_node = ...```?