tools: add xml_to_json module

[MyPyDavid]

No description provided.

[Zack-83]

Ich kann den Skript nicht testen, aber ich habe vollen Vertrauen in David :)

[MyPyDavid]

I've added it here because it was mentioned in the https://github.com/rdmorganiser/terms readme (tools/convert.py), but it was missing from here.
If you don't need it in here then I can move it to the terms repo as well..

[jochenklar]

I think @triole removed that by accident and nobody noticed: 3a04352

[triole]

Actually I removed it intentionally because I thought I was the only person using these scripts. I wanted to clean up the repository to simplify things and make them less confusing. Also I understood these scripts as a temporary solution and I am pretty sure the problem could be solved better. Nonetheless I am fine with you considering it as useful to restore these files.