Reorganize mapservice and validation files

https://github.com/geoadmin/mf-chsdi3/issues/42

No functional changes.

The purpose of this PR is to reduce the size of mapservices.py and validation.py by splitting them in smaller entities and therefore improve readability

mapservice.py is now splitted into

- layers.py
- features.py
(and filters.py where common filters are shared)

Validation classes have now one file per class in chsdi/lib/validation/

Some redundant code has also been removed.