Make CUDA optional dependency

[pelesh]

This PR reduces internal dependencies on CUDA SDK libraries. Classes MatrixHandler and VectorHandler are refactored to allow CPU-only or CUDA implementation of matrix and vector operations to be selected at runtime. With these changes:

• ReSolve can be built without CUDA
• Hardware backend (CPU or CUDA) is selected at runtime
• The code now builds on Linux and MacOS
• Fixed bug in FGMRES destructor

The build configuration allows for multiple hardware backends to coexist. In theory, we should be able to add HIP backend and build it at the same time as CUDA backend. If no GPU backend is built, a dummy device backend needs to be built instead (see CpuMemory.hpp).

Known issues not addressed in this PR:

• matvec function implemented in MatrixHandlerCuda class corrupts matrix descriptor object.
• There are memory leaks in unit tests for CUDA implementations of matrix and vector handlers as CUDA workspace is not deleted.

[cameronrutherford]

@pelesh PNNL GitHub supports MacOS and Windows runners. Can we add that for ReSolve in GitHub actions?

Additionally, now ReSolve supports building on Linux w/ Spack in GitHub actions, so we should add a pipeline and testing for those builds without CUDA.

[kswirydo]

I thought about it for some time and there was probably an easier solution to make all lin alg operatios (in VectorHandler and MatrixHandler) run with various backends, but it would had been a CMake nightmare and not necessarily backward compatible. I think the stuff in this PR is pretty good. I had some minor comments. Also, why IndexPlusValue changed its name?

[cameronrutherford]

With better docs will come enlightenment. LGTM

EDIT: We should make issues for:

• Future work
• Known bugs
• CI/CD additions

[ryandanehy]

@pelesh PNNL GitHub supports MacOS and Windows runners. Can we add that for ReSolve in GitHub actions?

Additionally, now ReSolve supports building on Linux w/ Spack in GitHub actions, so we should add a pipeline and testing for those builds without CUDA.

Will that mean we have to setup a mirror to a PNNL github given this under ORNL?

[pelesh]

@pelesh PNNL GitHub supports MacOS and Windows runners. Can we add that for ReSolve in GitHub actions?
Additionally, now ReSolve supports building on Linux w/ Spack in GitHub actions, so we should add a pipeline and testing for those builds without CUDA.

Will that mean we have to setup a mirror to a PNNL github given this under ORNL?

I think so.

[cameronrutherford]

@pelesh PNNL GitHub supports MacOS and Windows runners. Can we add that for ReSolve in GitHub actions?
Additionally, now ReSolve supports building on Linux w/ Spack in GitHub actions, so we should add a pipeline and testing for those builds without CUDA.

Will that mean we have to setup a mirror to a PNNL github given this under ORNL?

Mirror from ORNL GitHub to PNNL GitLab should be sufficient without PNNL GitHub in between