Snntoolbox

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Added support for Tensorflow 2.2.
The toolbox no longer imports stand-alone Keras, but instead uses Keras only
from within Tensorflow (tf.keras).
Enabled simulating SNNs in the tensorflow-based INIsim using graph mode rather
than eager execution, which results in a speed-up of about 7X.
Removed support for python 2.
Updated various temporal coding backends.

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The toolbox now supports input models from the PyTorch library.

Thanks to Pengfei Sun for contributing.

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The toolbox now supports deploying converted networks on the SpiNNaker
architecture!

Thanks to ej159, pabogdan, and rbodo for contributing.

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Simulation with Brian2 backend now supports:
- Constant input currents (less noisy than Poisson input)
- Reset-by-subtraction (more accurate than reset-to-zero).
- Bias currents

Thanks to wilkieolin for this contribution.

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Bugfixes:
- Setting biases in convolution layers for pyNN and Brian2 simulator
backends.
- Parsing axis parameter in BatchNorm layers.
- Counting of SNN operations.
- Minor issues due to updating to latest keras / tensorflow version.
- Syntax error in equation for membrane potential due to updating Brian2.
- Restoring a previously saved SNN to run with INIsim now works again.
- Fixed issue 25 (permutation of weights after flatten layer in models
trained with recent Keras version and simulated with Brian2 / pyNN).

Added support for:
- Intel's neuromorphic platform "Loihi".
- Tensorflow 2.0.
- Parsing depthwise-separable convolutions.
- Strides > 1 for pyNN and Brian2 simulator backends.
- Parsing a model can be skipped now by loading a previously saved parsed
model.
- Using SNN toolbox more easily from within a python script instead of via
terminal only.
- Save and load functions for Brian2 networks.

Miscellaneous:
- Added end-to-end examples for creating and training the model, saving
the dataset, and setting up the config file to run SNN toolbox.
- Moved large model files and datasets to separate repository
(snntoolbox_applications) to shrink size of core package.
- Minor refactoring, repo cleanup, and performance improvements.

Contributors:
- rbodo
- sflin
- nandantumu
- morth
- wilkieolin
- Al-pha