Cherry-pick of "Merge branch 'klf-inclusion' into 'master'" into 1.0

Add KLFitter to atlasexternals

See merge request !229 (merged)

(cherry picked from commit 3e11741f)

• c4254bfd Added very basic cmake configuration for KLFitter
• f01df4ee Added KLFitter to the AnalysisBaseExternals package filters
• 33356835 Added KLFitter to the AthAnalysisExternals package filters
• 55a8461f Changed authentication for KLFitter repo to SSH
• 7cb2ede2 Update KLFitter version (now using cmake for compilation)
• 430d66cb Updated KLFitter version (fix strategies for finding dependencies)
• 125a5345 Updated KLFitter (located now on github, v1.0.0)
• acec2632 Make use of KLFitter cmake option to not build examples
• a3724a13 Fixed FindKLFitter module with correct include names
• 5430ed23 Removed outdated remark in comment section

This is actually what we'll need for the 21.2 branch of atlas/athena...