Arbor Library v0.8.1

# v0.8.1

** 2022 12 20 **

A 🎄 holiday release! Not much has changed in a month, but we'd like to share it all the same. Notably, the [Arbor GUI](https://github.com/arbor-sim/gui/) [is co-released](https://github.com/arbor-sim/gui/releases/tag/v0.8) as of Arbor v0.8, and v0.8.1 will be no different.

## Major new features

- Voltage Processes: add the VOLTAGE_PROCESS mechanism kind to modcc, allowing for direct writing to the membrane voltage (#2033)

- Spack gpu option: added conditional variant for cuda builds to enable GPU-based random number generation (#2043)

- SDE Tutorial (#2044)

## Breaking changes since v0.7

- None 💃!

## Bug fixed

- Fix ornstein_uhlenbeck example on gpu (#2039)

- Setting ARB_MODCC was broken and nunfunctional. Fixed. (#2029)

- The `--cxx` flag in `arbor-build-catalogue` is now properly used; falls back to `c++`. (#2051)

## Full commit log

* Post release: add Zenodo entry, add Spack entry, update docs and scripts by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2036

* BUGFIX: add ARB_CUDA flag to example catalogue by @boeschf in https://github.com/arbor-sim/arbor/pull/2039

* Additional builtin functions to Arbor's NMODL dialect by @boeschf in https://github.com/arbor-sim/arbor/pull/2035

* Throw better errors when we cannot look up ion diffusivity by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2040

* ⚡ Voltage Processes by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2033

* simplify make catalogue by @boeschf in https://github.com/arbor-sim/arbor/pull/2042

* spack gpu option by @boeschf in https://github.com/arbor-sim/arbor/pull/2043

* Remove deprecated spike_detector. by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2041

* make ARB_MODCC functional again by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2029

* Yank v0.5 from Spack file, it does not build due to change in setting arch by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2037

* Use pugixml instead of libxml2 by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2048

* [BUGFIX] a-b-c: actually set compiler, improved default by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2051

* Sde Tutorial by @boeschf in https://github.com/arbor-sim/arbor/pull/2044

* Set Python to known version for all CI workflows by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2058

* consistent mechanism ids by @boeschf in https://github.com/arbor-sim/arbor/pull/2057

* Fix docs about exprelr. by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2064

* Allow __ in profiler names. by @thorstenhater in https://github.com/arbor-sim/arbor/pull/2065

* CI fixes for wheel building, prep for musllinux, Spack by @brenthuisman in https://github.com/arbor-sim/arbor/pull/2069