Prepare release 2.2.1

[eobermuhlner]

• add release number header to release note
• rename release note
• create empty release note for next release from template_release_note.md
• change version in build.gradle
• upload artifacts to maven central
  • run ./gradlew clean
  • run ./gradlew :ch.obermuhlner.math.big:uploadArchives
  • go to https://oss.sonatype.org/
  • in tab 'Staging Repositories' locate own Repository (typically at the end of the list)
  • verify content of own Repository (version number!)
  • Close own Repository
  • Refresh until Release becomes enabled
  • Release own Repository
• create github release from same artifacts
  • Create new draft release
  • Copy content of release note into draft release
  • Add artefacts from gradle build to draft release
    • big-math-*.jar
    • big-math-*-javadoc.jar
    • big-math-*-sources.jar
  • Verify all changes are committed and pushed
  • Publish release
• update readme
• add generated javadoc to docs/javadoc
• update docs/index.md
• update dependent projects
• create regression project for new release
  • edit new build.gradle to use new release library
  • edit settings.gradle to add new regression project
  • run ./gradlew :regression.v2_2_1:run (verify version number) with standard machine
  • edit analysis/*.csv files to add new release numbers
  • run csv2chart --property chart=line *.csv in directory regression/analysis