Keep DNP/NOPOP in output with Qty=0 and column DNP

[mdeweerd]

To make sure that no component was forgotten during the analysis, it is common practice to also list the DNP/NOPOP components in the BOM file on separate lines with quantity 0 and a DNP/NOPOP mention somewhere.

So the feature would be to group all DNP/NOPOP components at the end of the list - with Qty 0 and possibly with the pricing info (not required).
Qty 0 is already an indication that the component is not mounted, but an extra DNP column may be usefull for some.

[hildogjr]

First study:
This should be activated by a --show_dnp and may be required some changes in the group routines. Also some implementation to make sure that will be show at the end of the spreadsheet.
(Files to be modified: each BOM read module, spreadsheet.py order and group routine).

A important question is: if one DNP part is grouped with other of same kind but that have to be assembled?

[hildogjr]

Needed #359 fix for not workaround.

[mdeweerd]

Stale, closing (limiting my issue backlog).