Exception when using map_triggers with mfli

[THuckemann]

map_triggers complains about the mfli (and probably the SR830) not having an _is_triggerable attribute.
This occured only on Selene but works fine on Sionludi. As map_buffers is called before _map_triggers the latter might be skipped as the MFLI (being a buffered instrument) is usually mapped already when _map_triggers is called.

Simple fix would be to make the is_triggerable method return False when the object doesn't have the _is_triggerable attribute.

I'll have to reproduce the problem on another setup in order to check the fix...