Major changes

Fixed numpy.memmap concurrent accesses. Which fixes many issues with parallelization. The memmap name is now unique for each buffer.
Fixes #141, fixes #112, fixes #57
Sparse matrix initializers in mat_gen can now take a degree argument instead of connectivity.
Delay node, which returns its input with a specified delay.
Complete reimplementation of the ScikitLearnNode introduced in ReservoirPy v0.3.10. This has breaking changes, as the previous implementation was not consistent with the library node API. The tutorial has subsequently been updated.
Dropped support for Python 3.6. Python 3.7 should still work though it reached end-of-life.

Minor changes

dataset.narma now takes a u parameter so that the input timeseries can be accessed. Fixes #142
Fix : The ESN node now has consistent results across backends
Creating a Reservoir node with incorrect argument now raises a ValueError. Fixes #138
Fix: the default Node.partial_backward method of offline nodes didn't concatenate output timeseries correctly.
Many small corrections in documentation. In particular:
- Fix #134
- Fix #111
- Added documentation for the Concat node