More concise CSV header format for nodes and relationships

where header entries with semantic data, like node ids, relationship type
and labels must be specified with their corresponding types, no longer
relying in their position in the header. At the same time removed the need
for naming these columns.

Previously some header entries relied on position and some relied on type
being specified.

So, now a node header might look like:
:ID,name,:LABEL,year_of_birth:int

and a relationship header like:
:START_ID,:TYPE,:END_ID,some_property:String