ProxSuite 0.2.6

What's Changed

• User option sparse backend by @fabinsch in #84
• CI for c++14 compatibility by @fabinsch in #88
• Update README.md by @fabinsch in #89
• CI: use v1.2 also for windows, add cxx_std to key by @fabinsch in #90
• Fix heap allocation in dense solver by @jcarpent in #92

Full Changelog: v0.2.5...v0.2.6