[FIX][SCHEMA] Make recommended consistent

[AugustijnVrolijk]

recommended_fields -> recommended. To make it more machine readable..

[AugustijnVrolijk]

Somewhat unrelated. For metadata of "type: object". Would people consider moving required/recommended fields for objects with properties to objects.enums. Or objects.metadata?

Making this consistent would make it more machine readable, and allow for treatment of property keys as "strict" objects with an easily findable definition (i.e. shema.objects.metadata.get(name)).

This is sometimes done, i.e. for StimulusPresentation:

StimulusPresentation:
  name: StimulusPresentation
  display_name: Stimulus Presentation
  description: |
    Object containing key-value pairs related to the software used to present
    the stimuli during the experiment.
  type: object
recommended:
    - OperatingSystem
    - ScreenDistance
    - ScreenRefreshRate
    - ScreenResolution
    - ScreenSize
    - SoftwareName
    - SoftwareRRID
    - SoftwareVersion
    - Code
    - HeadStabilization
  properties:
    OperatingSystem:
      $ref: objects.metadata.OperatingSystem
    ScreenDistance:
      $ref: objects.metadata.ScreenDistance
    ScreenRefreshRate:
      $ref: objects.metadata.ScreenRefreshRate

But othertimes ignored, i.e. for DeidentificationMethodCodeSequence:

DeidentificationMethodCodeSequence:
  name: DeidentificationMethodCodeSequence
  display_name: Deidentification Method Code Sequence
  description: |
    A sequence of code objects describing the mechanism or method use to remove the Patient's identity.
    Corresponds to [DICOM Tag 0012, 0064](https://dicomlookup.com/dicomtags/(0012,0064))
    `De-identification Method Code Sequence`.
  type: array
  items:
    type: object
    recommended_fields:
      - CodeValue
      - CodeMeaning
      - CodingSchemeDesignator
      - CodingSchemeVersion
    properties:
      CodeValue:
        name: CodeValue
        type: string
        description: |
          An identifier that is unambiguous within the Coding Scheme
          denoted by Coding Scheme Designator and Coding Scheme Version.
          Corresponds to [DICOM Tag 0008, 0100](https://dicomlookup.com/dicomtags/(0008,0100)) `Code Value`.
      CodeMeaning:
        name: CodeMeaning
        type: string
        description: |
          Text that has meaning to a human and conveys the meaning of the term
          Corresponds to [DICOM Tag 0008, 0104](https://dicomlookup.com/dicomtags/(0008,0104)) `Code Meaning`.
      CodingSchemeDesignator:
        name: CodingSchemeDesignator
        type: string
        description: |
          The identifier of the coding scheme in which the coded entry is defined.
          Corresponds to [DICOM Tag 0008, 0102](https://dicomlookup.com/dicomtags/(0008,0102))
          `Coding Scheme Designator`.
      CodingSchemeVersion:
        name: CodingSchemeVersion
        type: string
        description: |
          An identifier of the version of the coding scheme if necessary to resolve ambiguity.
          Corresponds to [DICOM Tag 0008, 0103](https://dicomlookup.com/dicomtags/(0008,0103)) `Coding Scheme Version`.

[effigies]

LGTM. Good catch.

[effigies]

Somewhat unrelated. For metadata of "type: object". Would people consider moving required/recommended fields for objects with properties to objects.enums. Or objects.metadata?

objects.metadata makes sense to me. They aren't really enumerated values, but subfields. Feel free to open a PR. I would do it against master, though, not maint/*.