Add verification on source upload

When a source_manager uploads a source, there is currently no verification on the quality of the file and its content.

There is a need to explicitly explain where there are errors with warnings etc.

Verify:

• That the file is json
• That the content follows IS standard
• That the mandatory variables are present
• That the variables from a controled list are correct
• That the variables type is correct (controled list, date, numeric, string)
• That the number of variable in a list follows its cardinality