1.2.0 2016/12/12 ABI 6:0:0
* New functionality: the plug-in from which each subcontractor is
registered is now tracked, so that it becomes possible to look
for a subcontractor matching a specific kind and plug-in. We
introduce the function Gyoto::requirePlugin() in addition to
Gyoto::loadPlugin(). The subcontractor signature is changed to
accept a second parameter, which breaks the API.
* New functionality: thread-unsafe classes declare themselves as
such, so that NThreads is ignored when multi-threading would
cause erroneous results.