This reverts merge request !105

Let's try to find all build problems in one try… keep building after
the initial error to find additional ones.

This should fix the error about an ambiguous template instantiation
for 'struct halco::common::detail::limits<
halco::hicann_dls::vx::v1::SynapseOnSynapseRow, void>' which isn't
triggered on newer g++s. We dropped support for old `v1` hardware; this
was planned anyways.

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.