metadata: Move the rootpwaDataFileWriter.

As the naming of the dataConverterUtils.hpp does not make sense anymore, move
it to dataFileWriter.h. To have a clean move, the file is left unchanged,
resulting in broken compilation in this revision. Will be fixed in the next
commit.
metadata: Fix compilation.

Fix compilation after the renames in the previous commit. Move most of the code
of the rootpwaDataFileWriter from the header file to the source file, fix the
cmake files, the includes and some other minor problems.
metadata: Rename rootpwaDataFileWriter.

Rename the rootpwaDataFileWriter to dataFileWriter, as the "rootpwa" was
completely unnecessary.
pyInterface: Add bindings for the dataFileWriter.

Add the python bindings for the dataFileWriter class. Using this, add a small
python script convertDataFile.py, which does the same as C++ version, just a
little less ironed out (less printout, debug switch does nothing, less error
checking).
pyInterface: Improve conversions of pairs.

Improve the conversion of std::pairs by using the function which was
implemented in the previous commit which facilitates and harmonizes conversions
form python tuples to std::pairs.