Support filtering and labeling by relationship type for item-piechart

[JasperCraeghs]

• Added :sourcetype: option to the item-piechart directive
• Added :targettype: option to the item-piechart directive
• Fixed a bug that resulted in using the <<attribute>> value of the nested target item that was parsed last as label instead of respecting the order of the arguments of the :<<attribute>>: option, or, in case of multiple target items, discarding the effect of those defined first.

Resolves #270

[JasperCraeghs]

Which label shall be used for an RQT-item that is linked to two TEST-items, of which only one is linked to a TEST_REP-item? covered or executed? The current implementation on master branch is not consistent and depends on the order in which the targets are defined.

.. item-piechart::
    :id_set: RQT TEST TEST_REP
    :label_set: uncovered, covered, executed

[bavovanachte]

I would say "covered" specifically in the case of test coverage. I haven't used the item-piechart directive yet, but from the documentation, I understand that "covered" in this case would mean that the REQ has a TEST linked, but not a test report.

I would think it's deceiving to mark a requirement as "executed" if only one of its testcases has been run. I'm guessing requirements with multiple testcases is not uncommon, and my opinion would be that a requirement is only covered if all of its planned testcases have been executed

[Juegadamas]

I'm with @bavovanachte. I'd say covered for the same reasons.