Spectra v0.9.0

Added

• Added support for CMake build, contributed by Guillaume Acke and Jens Wehner (#70, #88)
• Spectra can now be installed via conda-forge, thanks to Guillaume Acke and Julien Schueller (#81, #85)
• The source code of Spectra is now formatted using ClangFormat, suggested by Jens Wehner

Changed

• Fixed a compiler warning caused by unused parameter, contributed by Julien Schueller (#80)
• Changed the implementation of BKLDLT solver to improve precision in some tests