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...)
This would require changing *all* classes derived from ```XmlQtInterface```. The idea was to return an ```int``` to allow different error-codes to be returned. Currently we don't make use of this and always return just 0 or 1, so we could change the return value to ```bool```.
Should I do this here or later in a separate PR?