Update Cmake to allow third-party libraries

[kkiesling]

This addresses #1779. CMake will now search for existing installations of xtensor, xtl, and gsl-lite and use those instead. If not found, it'll use the vendored submodules.

[paulromano]

Thanks @kkiesling and congrats on your first PR here! 🎉 Just a few comments regarding how we handle xtl

[kkiesling]

I think I have at least one solution to one of the build errors coming from xtensor on CI (we'll see when it is finished running since I can't really reproduce this locally). For xt::flip I explicitly included the header file that it comes from.

For the other build error with xt::col, I think that comes down to a version issue. vectfit builds with xtensor version 0.21.2, which is then found as the already installed version, where as the OpenMC vendored version is 0.21.3. It looks like xt::row and xt::col were introduced in 0.21.3 (see pr). We could probably get around this by requiring a minimum version of xtensor and if it's not satisfied then install the vendored version instead.

[kkiesling]

One last change and hopefully this makes it work- I updated the xtl and xtensor versions on the CI build script for vectfit to be the versions that are vendored. This new find_package change was finding the versions installed with vectfit, but they were too old and didn't have some of the functions currently used by openmc.

[paulromano]

Looks good! Thanks for working through the strange build issues 😄 Will merge tomorrow morning if there's no more feedback