Switch madis to rely on pypy instead of python command

[marekhorst]

This feature should improve madis scripts performance and was requested on redmine: #4177#note-2.

Currently python commands defined in:

• workflow.xml files
• *DBBuilder.java java code
• classify_documents.sh script

rely on self-executable mexec.py. We should alter those references to provide the pypy command explicitly, so e.g. instead of:

scripts/madis/mexec.py -d project.db -f scripts/projects.sql

we should define:

pypy scripts/madis/mexec.py -d project.db -f scripts/projects.sql