tweak overload resolution of TupleObj::farray for gcc7

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@clemenci, @cattanem, @erodrigu: initial work around for gcc7 compilation -- very ugly, but it compiles. Should be functional as well, but would be good if there would be an explicit test that confirms that. I'll try to find a better workaround on a somewhat longer timescale.

Thanks @graven. @erodrigu unfortunately this comes too late for the Gaudi v29r0 stack that I have started to tag (see lhcb-prerelease nightly as from tomorrow). It means that DaVinci will have to wait until the Gaudi v29r1 release at the end of the year for gcc7 support...

In that case, I'll switch this to WIP, and take a bit more time to make a more elegant fix....

marked as a Work In Progress

Well... it would be nice to be able to run the DaVinci tests while waiting for the proper fix...

I can remove the WIP, under the assumption it won't be applied just yet (unless I give up). Given the rate at which Gaudi mr are merged, this is probably not something to worry about ;-)