Proxy: add special case for `container()` in scalar-mode gather2D-behavior

Gerhard Ravenrequested to merge
track-v3-scalar-gather2d-support into master

Required to allow eg. v3 tracks to be the target in a SOAcollections using an indices2D_field, eg.

struct ChildIndexArray : LHCb::Event::indices2D_field< LHCb::Event::v3::Tracks const , 2, 8 >{};

Should go with Rec!2749 (merged)

Edited by Peilian Li

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