v20.13.0

• Changed version file to 20.13.0
• Merge pull request #45 from m3g/xygauss
• update readme
• remove docs from master
• move docs to root
• create test index.html file
• Merge pull request #46 from SSchott/master
• Merge branch 'master' into xygauss
• Modified xygauss to avoid floating point exception
• removed old rest assignmnet
• add to f the result of xygauss below
• copied xygauss changes to new dir structure
• Merge remote-tracking branch 'upstream/master'
• Merge remote-tracking branch 'upstream/master'
• XYgauss gradient correction
• XYgauss comprest+getinp
• XYgauss surface