Support array and LAPACK features on MacOS and Windows.

[olson-sean-k]

The array feature uses ndarray and supporting crates to implement operations that require solving linear systems of arbitrary size. This allows Theon to optionally support certain useful spatial queries, such as computing a best-fit plane given a sampling of points.

This depends on ndarray-linalg, which builds and links BLAS/LAPACK libraries. These require building source packages and don't offer consistent support for different OSes. In the worst case, the output on different OSes differs (which is very likely a bug). 😬 Currently, this feature is gated behind cfg(target_os = "linux").

Only supporting certain OSes means that downstream code must always use conditional compilation. If possible, find some combination of packages that build and run on Linux, MacOS, and Windows.

[olson-sean-k]

As mentioned in #9, the different output on different OSes is probably due to bad tests. The determined plane test uses collinear points!