Publish Palace CPU package to an index for optional installation

[vvahidd]

PyPI rejected the gsim 0.2.0 release because the palace-toolkit-cpu extra used a direct GitHub wheel URL in Requires-Dist metadata. PyPI does not accept direct URL dependencies, including optional dependencies.

The immediate release fix removes the extra and documents installing the Linux x86_64 wheel directly. This means pip install gsim[palace-toolkit-cpu] is no longer available.

@mdmaas i had to remove this dep to release a new version. either we have to absorb palacetoolkit-palace-cpu in here or release that on pypi.

[mdmaas]

Oh yeah, the same thing happened in palace-toolkit. I think the solution there was to implement the download step at runtime in setup.py.

We could certainly absorb the CI commands from palace-toolkit-cpu that compile palace from source on CI, and that could also help with keeping the gcloud server version on sync with the local one (see #203 where it is indicated that we need to change the compiler flags on the gcloud palace version).

[benvial]

There is this package I made for local palace https://github.com/benvial/palace-solver
it is pip installable, linux only for now