qonnx version 0.2.0

What's new in this release

• Support exporting QONNX from QKeras (#28 )
• Support exporting QONNX from HAWQ (#41)
• Tensor attributes in custom op wrappers (#24)
• Use knowledge of weight matrices to further optimize accumulator range calculation (#29)
• Support spatial -> channel rebalancing for certain convolutional kernels (#22)
• Numerous bugfixes and other improvements - see full list of merged PRs for this release