v1.5.6

• v1.5.5 skipped due to issue with publishing tool.
• Allow datasets to specify HED versions or individual files as validation schema
  enhancement
• Fix type column checking in *_channels.tsv files.
• Add EthicsApprovals to dataset_description schema.
• Validate optional fractional seconds in AcquisitionTime in scans.tsv
• Add _recording matches to continuous measurement extensions where applicable in file_level_rules.json.
• Introduce common schema for shared JSON fields, make schemas stricter.
  enhancement
• Implement HED value-taking strings.
• Simplify dataset_description schema, enhance phsyio schema.
• Fix cross-talk and fine-calibration extensions.
• Allow top level sbref, json for dwi modality.
• json schema: PowerLineFrequency must be number > 0
• add participants.json to valid_dataset testing data
• Cast dataset desc author values as string when testing for commas.
• Update top level file regex to allow modality metadata files with no required entities.
• Update how we generate filenames of potential sidecars.
• Extend validator for crosstalk+finecalib for FIF
• Update link to contributors
• Allow string n/a for PowerLineFrequency in EEG,MEG,iEEG
• Update contact in Code of Conduct to @franklin-feingold
• Make beh and func file level rules consistent with spec.
• Check continuous recording datafiles have matching json.
• Allow optional UTC specifier Z in aqc_time.
• Bump elliptic from 6.5.0 to 6.5.3
• Bump handlebars from 4.1.2 to 4.7.6
• Bump codecov from 3.6.5 to 3.7.1
• Bump lodash from 4.17.14 to 4.17.19