Fix pynn-brainscales build 3 (and revert to rc2)

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!106

Declare mpi variant in PyNN package

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!100

This reverts merge request !105

Fix pynn-brainscales build #2

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!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

Fix pynn-brainscales build (and revert to rc2)

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!104

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

This reverts merge request !103

Fix pynn-brainscales version specification (and revert to rc2)

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!103

This reverts merge request !102

Update pynn-brainscales

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!102

Enable oPPUlance support in pynn-brainscales

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!101

We can drop oppulance from `spack.yaml` as it is no top-level
dependency anymore.

(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.)

Upgrade oPPUlance (cross compiler for BrainScaleS-2's embedded "PPU" processor)

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!99

This reverts merge request !87

Update PyNN

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!87

Added biobb-md

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!93

Fix py-torch: Enable tensorpipe

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!96

Add py-frites

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!97

Add py-viziphant

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!91

Add py-elephant

See merge request technical-coordination/project-internal/devops/platform/ebrains-spack-builds!90