pylib: first attempt with sqlalchemy ORM

This is a first draft of the mappying class with sqlalchemy
Generated with https://github.com/lclementi/sqlautocode
plus several manual edit:
 - default are not set
 - several data type changed to the default SQL (instead of mysql)

TODO:
 - define reverse mapping for the foreign keys (see sqlalchemy.orm.relationship)
 - Add views
 - Add stored procedure
pylib: test code for sqlalchemy to verify current mapping classes

Some scratch code used to test the mappings
gitignore: add a gitignore to make status more readable