ENH: Add a debugging shape test to Node._message_to_child

In order to detect bugs, a simple test was added to
Node._meesage_to_child. This test checks that the variable axes and the
plate axes of the message (i.e., moments) are proper.
BUG: Slice node modified the moment list of the parent node

Slice operator node accidentally modified the moment list of its parent
when computing the moments. This is now fixed by sending a copy of the
moment list from stochastic nodes and by taking a copy of the moment
list of the parents when receiving messages from them. Also, a test is
added which makes sure that this works for the Slice node.
ENH: Implement and improve speed-up rotations for Gaussian MCs.

The speed-up rotations are now implemented for both Gaussian Markov
chains and drifting Gaussian Markov chains.
DEMO: Update the demo for drifting linear state-space model

The demo can be found at bayespy/demos/demo_lssm_drift.py. It can be run
from command line and given some arguments.
MAINT: Clean code a bit by removing some deprecated parts