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.)

This reverts merge request !87

If starting from scratch the archive file isn't there in do_fetch;
in this case, we can make use of the folder where the freshly
downloaded resources are cloned to.

Add py-neo, which is a dependency for py-elephant and py-pynn. An older version is available in upstream Spack.

Add py-quantities, which is a dependency for py-neo. This same version is available in upstream Spack, adding it here in case we need to add new versions quickly

Dependency on py-pybind11 now "open" interval… (i.e. later than 2.6.2
allowed for newer version of pytorch).