Small fixes and improvements

• Minor Enhancements

  • Allow pasing singular Symbol to CSV converters option (@takkanm)
  • Support calling GroupBy#each_group w/o blocks (@hibariya)
  • Refactor grouping and aggregation (@paisible-wanderer)
  • Add String Converter to Daru::IO::CSV::CONVERTERS (@takkanm)
  • Fix annoying missing libraries warning
  • Remove post-install message (nice yet useless)

• Fixes

  • Fix group_by for DataFrame with single row (@baarkerlounger)
  • #rolling_fillna! bugfixes on Daru::Vector and Daru::DataFrame (@mhammiche)
  • Fixes #include? on multiindex (@rohitner)