This reverts merge request !104

Includes a missing header fix in hxcomm@ebrains-2.0-rc3

This reverts merge request !103

This reverts merge request !102

(BSS-2 features embedded SIMD processors for plasticity and other use
cases. This prepares for the next pynn-brainscales update adding
some code-generation features.)

(Before we somehow used the runtime dependency on py-pyyaml.)

That's the first version that should really work :); rc1 experienced
scheduling problems when multiple users were accessing the same hardware
resource in parallel.