v0.18

• Added support for Cytokine data with the Cytokine class and associated test
  and example scripts.
• Added support for HostSeqSet, HostTranscriptomicsRawSeqSet,
  MicrobTranscriptomicsSeqSet, Lipidome,
  Metabolome, ClusteredSeqSet, ViralSeqSet data with the classes and
  associated tests and example scripts.
• Added metabolomes(), lipidomes(), cytokines() and children() support for
  HostAssayPrep and MicrobiomeAssayPrep classes.
• Some logging improvements.