Sketchlib v2.1.0

• Remove armadillo from the code entirely in favour of Eigen
• Actually link BLAS (used in the regression), LAPACK not actually needed
• Use a new regression solver, see 'Normal equations' under https://eigen.tuxfamily.org/dox/group__LeastSquares.html
• Solve issues with bool in highfive code, now uint8_t explicitly used
• Use KMeansRex, as the kmeans in armadillo is now removed

nvcc now sees the eigen code and throws up a few warnings, but this doesn't seem to be causing as many problems as it used to

What's Changed

• BLAS linking; CI fix; better message for deprecated function by @johnlees in #84

Full Changelog: v2.0.1...v2.1.0