Build core functions as (static) library, add testing

[nzfeng]

This PR

• Packages the core functions into a library, to make inclusion in other projects easer
• Checks for AMGCL issues, and falls back to the original direct solver if necessary
• The demo program & data now lives in a separate sample project demonstrating how to incorporate the library into a CMake project
• Removes the rebuild solve option; now rebuilding is automatically determined to be false within the solver if solve parameters differ from those used last
• Removes Polyscope as a dependency, which was only necessary for the sample project
• Adds tests
• Adds documentation for both C++ and Python using mkdocs-material