Make some attributes optional

[RomainTT]

Maybe it was intentional that attributes are mandatory. But I think some of them can have a default value to null.

If one does not want to check the filename, one does not necessarily want to write the whole filename object in my schema.

min and max could also have a default value equal to null.

I think the following attributes must be mandatory:

• name (for the main object and colum objects)
• type (for colum objects)
• columns (for the main object)