Observation Unit Level only allows specific set of observation level types

[ctucker3]

The observation level class is an enum that only allows for the following types:

STUDY("study"),
  FIELD("field"),
  ENTRY("entry"),
  REP("rep"),
  BLOCK("block"),
  SUB_BLOCK("sub-block"),
  PLOT("plot"),
  SUB_PLOT("sub-plot"),
  PLANT("plant"),
  POT("pot"),
  SAMPLE("sample")

The brapi spec does not restrict you to these options, these are just the recommended types. Change the observation level name to just be a String.