Improve dd_helpers.py to avoid relying on Saxon JAR

[olivhoenen]

To simplify the code base, we should modify the dd_helpers.py to use saxonche directly in Python instead of downloading and using the Saxon-HE JAR file from an external process. It may also improve the performance of the XSL transforms.

(this was done already in imas-data-dictionary )

[olivhoenen]

Merge in develop together with #14