Upgrade to Spack v1.0.0

[nkoukpaizan]

Description

Upgrading to upstream Spack v1.0.1 and using a Spack repo for the yet-to-be-pushed-upstream GridKit package. As of Spack v1.0, the core library (https://github.com/spack/spack) is separate from the packages (https://github.com/spack/spack-packages), which simplifies the use of a local repo to keep changes that we need.

Closes #77
Closes #187

Proposed changes

• Buildsystem-only changes.
• Pointing to upstream spack instead of my fork
• Local Spack repo to maintain the GridKit package. We will need to contribute this upstream eventually.
• Updated CI accordingly

Checklist

• All tests pass.
• [N/A] Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• [N/A] The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.

Further comments

[shakedregev]

Looks good, some semantic changes.