Merged PR 1583: ELL release v3.1.0

ELL release v3.1.0

- Move to VS 2019
- Fix a codegen error that was resulting in incorrect functional behavior
- Fix regressions in audio training tutorial (#232)
- Add importing of Sum nodes to ONNX importer
- Fix crash in LLVMContext::SetName
- Improved performance of CNN models on Pi3 with new implementations of spatial, pointwise and regular convolutions
- Improved performance of reorder node
- New nodes: ReorderDataCodeNode, SpatialConvolutionNode, MatrixMatrixMultiplyCodeNode
- Implement parallelization strategies for matrix multiplication nodes.
- Only enable new MatrixMatrixMultipleCodeNode path for select ARM targets like Pi, and not Intel/AMD CPUs
- Add the flag `--skip_ellcode` to `compile` and `wrap.py` tools to use OpenBLAS for linear algebra computations.

Merged PR 1373: ELL release v3.0.2

- Add FastGRNN node to ELL, train_classifier.py and the onnx_importer.

Merged PR 1356: ELL release v3.0.1 - History.md update

Merged PR 1310: ELL release v3.0.0

Merged PR 1283: ELL release v2.5.4

Merged PR 1270: ELL release v2.5.3

Merged PR 1268: ELL release v2.5.2

push new version to github

Merged PR 1238: Update History.md before release

Merged PR 1180: Add versioning info to ELL