Refactor and test external weights support

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

Adds an end-to-end test for the UNSW-NB15 (cybersecurity/network intrusion detection) MLP. Also includes a minor fix to the PYNQ driver (using correctly-generated tensors with respect to datatype for throughput testing)

includes trained model + Python validation script

Fix incorrect shapes for data packing in Pynq driver

Updates to end-to-end cybersecurity examples

Adjust notebook to changes in bnn-pynq brevitas example