needtable colwidths only supports relative widths

[ubmarco]

The colwidths option of the needtable directive should support the same features as
as the docutils listtable directive.

This however is not implemented, only values without unit are allowed, so the result will always be relative widths:

leads to

The issue needs to be resolved here
https://github.com/useblocks/sphinx-needs/blob/master/sphinx_needs/directives/needtable.py#L70