Document possibly unexpected side effect
Add and document side effect

Also some minor POD fixes.
Added/updated some tests
Rudimentary method tests added
Simplified some tests
CSI can be determined automatically

If we store some value for the code structure identifier (CSI)
we later on might need to update it, when other data elements
get added or removed from the item and contribution segments.
Instead we simply derive the correct value from the presence
of the respective elements.
Attribute csi has been replaced with a method
Rewritten the parser and changed return values