ENH: Add a debugging shape test to Node._message_to_child
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.

bayespy/bayespy,SalemAmeen/bayespy,jluttine/bayespy,fivejjs/bayespy