when a value is composed only with space tabs etc consider it as None

Now slim is able to create an instance, run make all, and start an interactive python without errors
refs #145

- Written a new custom_list validator, it takes another validator name as argument (in order to validate items of list)
- Using it to validate list of plugins names

Allows runtest to run again =)

Able to :
- create/delete instances
- list instances
- run make %target% in all instances
- edit configuration given an instance name

refs #145

Nescessary for refs #145

Implements the check of the bases classes of a datasource class in a datasource plugin ( solves #144 )

refs #132

solves #58

The file is the pyconfoigure m4 macros patch that solves tyhe python3 bug and add macro to check a module version number ( refs #140 )

Now backref seems to work at creation of a Person linked with a text the Person UID is appended to Text.linked_persons

Triggered a bug in wrong Integer DH tests

Return the value of this datahandler when empty

refs #131

The dedup mechanism was using a cast of list to set. But sometimes, filters contains list, and list are not hashable...

refs #131