Feature/fix streamline past eltwise add

[TEST] Add test linear_past_eltwise_add when the graph has multiple joins where to apply the transformation (was failing in this case) plu test moving linear tensor ops, not only scalar ones

[TRANSFORM] Generalize from past scalar to past any linear op (this was supported, but restricted). Fix node sorting problem, by iterating over a copy list of the node instead of iteration over graph.node. Fix corner case when both branches are empty and come from the same node

transform that moves scalar linear operations past eltwise add

Added Addstreams HLS Op

Implemented LabelSelect (TopK) HLS Op

LabelSelect is currently broken for signed inputs
will open issue separately

turns out there was a problem with signed values after all
failing testcases with k=5 produce the indices of positive values correctly, but negative ones don't seem to be compared correctly (always producing index 0?)

Implemented GlobalAccPool HLS Op

Merge branch 'feature/accpool_batch' of https://github.com/quetric/finn into quetric-feature/accpool_batch

Feature/vectorized multithreshold fxn