Move synaptic intelligence out of alpha stage

Synaptic intelligence `avalanche/training/plugins/synaptic_intelligence.py` currently has the following warning
```
warnings.warn("The Synaptic Intelligence plugin is in an alpha stage "
"and is not perfectly aligned with the paper "
"implementation. Please use at your own risk!")
```
As a researcher I would like it to be aligned with the paper and have appropriate tests to verify that.