Add KLU feature

[martinjrobins]

Fixes #3. This PR:

• adds klu feature
• adds SuiteSparse submodule (https://github.com/DrTimothyAldenDavis/SuiteSparse)
• if klu feature is added, then bindings are added for the sundials KLU linear solver (requires both sundials and suitesparse to be installed)
• If a vendor build, also builds and links suitesparse (using the SuiteSparse submodule) to the sundials library build (requires a BLAS installation to be present)

Note: I havn't added the klu feature to the CI yet. If would be good to get some feedback on how this should be tested. Should klu be a default feature, or should this be tested separately? Also, the current CI has an issue that I noticed: #5

[Chris00]

Thanks for your PR. I builds fine with the system Sundials. I see two issues we need to resolve.

• I think the paths for local build are not correct (cargo build --features klu fails) and one should act an extra environment variable for SuiteSparse — the default should be assuming it is installed alongside Sundials ($D/sundials/ and $D/suitesparse/) and not as a sub-directory. I can fix these but these paths may change due to the second issue.
• SuiteSparse repository is 268Mb while a crate can at max be 10Mb — and most of the 10Mb are already taken by Sundials (stripped of its doc, examples,...). So suitesparse should be dynamically downloaded (say with gitoxide but an additional large download may be frowned upon, especially because it will increase the size of each target/) or one must require the user to first install SuiteSparse and point to its installation with an environment variable.

[Chris00]

For the first point, I meant with the vendor libraries; cargo build --features klu,build_libraries to force it.

[martinjrobins]

regarding the 2nd point: you're right, I didn't think about the size of the submodule. I'm happy with requiring the user to have a local installation of suitesparse and pointing to it via an environment variable. I think this is a reasonable requirement for an optional feature. I can make this change to the PR if you're also happy with this?

[Chris00]

I pushed a commit enabling the KLU feature (in the dev branch). May you check that it works for you? If so, I'll make a new release.

[martinjrobins]

The dev branch works perfectly for me, thanks @Chris00!

[Chris00]

Thanks. I have released version 0.5.