Made QuantIdentityHandler and FoldQuantWeights compatible with DebugMarker nodes and added QONNX to the test_brevitas_debug test.

Fixed a bug in the QuantReluHandler, where the export would fail if the activation was at the top of the network.

Raised default for max_multithreshold_bit_width from 4 to 8, due to missing quantization for 8-bit input test networks (CNV and mobilenet).

Made QONNX to FINN a default build step, changed verification step name and made running of conversion conditional on the existence of quant nodes.