Module XML.pm merged into parser.pm

In the next version, the code from the module XML.pm will be merged into parser.pm. This simplifies some code and maintenance. This change also removes the "internal interface" between them, a variable containing the parsed data. The use of this variable other than by XML.pm was never intended and advised against, its structure undocumented and susceptible to changes without notice and backwards compatibility. If you are using this variable and need help with switching to the callback mechanism, please say so. In the request, please specify how the data should be provided to fit your software best, and if any of the examples in metaf2xml.pl comes close to your expected solution.