Check CSV well-formedness on initial load

[Langenfeld]

We had problems with ill formatted CSV imports sin the past such as an empty requirements Id, inconsistent line length (number of items in one row). Thus we want to check the input file for problems on loading:

• are all rows equally long (as long as the top row with titles)
• is the ID field (requirements ID chosen during import) empty
• are all IDs unique
• sluggify the IDs for file system usage
• check if type and description are None and replace by empty string, as this may be empty, but shall not crash