Alexander Ziegler / gammalib

… methods from GTools.

…parameter in an XML file.

… "xml_has_par" function and made reading of information in observation definition file optional (so that for example energies or ROI can be missing).

The XML module now handles predefined entities. Special characters, such as " will now be transformed into their predefined entities, e.g. ", when writing the XML file. Upon reading, predefined entities will be converted into special chacaters, so that the user always deals with a human readable string.

The gammalib::filepath combines a path name and a file name into a string. If the path name is empty, the file name is returned.

(I chose `centre` because there were more `centre` than `center`
around.)

The coding and design conventions (CDC) document was also updated.

Python has internal type conversion functions for instrinsic data types, so these are not needed here. One of the motivations of this removal was that touint() did not compile using swig 2.0.7 on Python 3.2.3.