ChangeLog update

Added jupyter notebooks with a guided tour through TATi.
Added Dockerfile for putting TATi in docker container.
Added analysis capability for ensemble averaging.
FIX: SGLD formula in userguide was incorrect.
Added GradientDescent optimizer with Barzilai-Borwein learning rate picker.
Accumulation of norms of gradients, noise, momentum, ... is made optional
and can be fully switched off for performance reasons.
Optimizers have been moved into distinct module/folder.
several smaller fixes preventing the distributed tarball from compiling.
small runtime improvements.
FIX: time_per_nth_step column in run info file/dataframe was wrong when
using HMC.

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