add developer flag and ignore stages in PDB starting with "_"

Yuma Uematsurequested to merge
feature/ignore-stages-under-development into devel-2.2

To hide stages in PDB under development, we propose to

  • add "_" at the head of the stage code in PDB
  • define global developer_flag
  • When downloading from PDB, ignore the stages whose code starts with "_" unless developer_flag is True

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