Add an implementation of a VAE for time series

[JunodCharlie]

Add a Variational Auto Encoder for anomaly detection to the models. While VAEs are fairly common, they are not often used with time series. Providing such a model would be greatly beneficial.

To do:

• [x] write the specification of the model (configuration options, underlaying library etc.)
• [x] add a VAE class in core/model
• [x] add the implementation
• [x] test and validate the model