PAGEOM_ — PAGE-OM issue: most attributes should be optional

All attributes are now mandatory in the model, but most of them should
be optional.
For example creation_date in Identifiable.

• Change the model in order to define the following attributes either optionally or mandatory. Then generate new documentation (chapter 7).
  The following attributes have been kept mandatory (they are listed here for information; there is no change here):
  Boolean.value
  Db_xref.uid
  Evidence_code.code
  Float_value.value
  Identifiable.lsid
  Integer_value.value
  Location_on_plate.x
  Location_on_plate.y
  Ontology_term.term
  Reference_genomic_location.start
  String_value.value
  Time_accuracy.code
  Unit.type
  Value_range.max
  Value_range.min

The following attributes have been changed to optional:

Abstract_population.ethnicity
Abstract_population.language_family
Abstract_population.primary_language
Abstract_population.race
Algorithm.description
Annotation.name
Annotation.value
Article.first_page
Article.last_page
Assayed_genomic_genotype.assay_failure
Assayed_genomic_genotype.quality_score
Bibliographic_reference.date
Bibliographic_reference.format
Bibliographic_reference.language
Bibliographic_reference.rights
Bibliographic_reference.title
Bibliographic_reference.type
Bibref_description.abstract_type
Bibref_description.language
Bibref_description.table_of_contents
Bibref_description.the_abstract
Bibref_description.toc_type
Bibref_scope.spatial_location
Bibref_scope.temporal_period
Book.edition
Book.isbn
Book.series
Book.volume
Conclusion.p_value
Constraint.description
Contributor.date
Db_xref.db
Db_xref.db_version
Db_xref.field
Db_xref.url
Defining_feature.type
Entry_status.last_modified_date
Entry_status.subset
Frequency.count
Frequency.value
Gene_product_change.label
Genomic_allele.bin
Genomic_allele.repeat_count
Genomic_allele.repeat_is_exact
Genomic_gene_structure.gene_symbol
Genomic_variation.flank_down
Genomic_variation.flank_up
Genomic_variation.is_mutation
Genomic_variation.repeat_unit
Genomic_variation.type
Genomic_variation.validation_status
Genotype_phenotype_correlation_experiment.objective
Genotype_phenotype_correlation_experiment.outcome
Genotype_phenotype_correlation_experiment.study_id
Genotype_phenotype_correlation_experiment.type
Geographic_location.max_latitude
Geographic_location.max_longitude
Geographic_location.min_latitude
Geographic_location.min_longitude
Hypothesis.description
Identifiable.creation_date
Identifiable.delete_date
Identifiable.modify_date
Identifiable.name
Individual.birth_date
Individual.death_date
Individual.father_id
Individual.gender
Individual.mother_id
Journal.abbreviation
Journal.issn
Journal_article.issue
Journal_article.issue_supplement
Journal_article.volume
Molecular_sample.molecule
Observed_value.time
Ontology_source.ontology_URI
Ordered_location.position
Panel.count_unit
Panel.pooled
Panel.size
Panel.type
Patent.doc_number
Patent.doc_office
Patent.doc_type
Person.affiliation
Person.email
Person.et_al
Person.first_name
Person.mid_initials
Person.postal_address
Person.surname
Reference_genomic_location.end
Reference_genomic_location.strand
Reference_genomic_location_in_assembly.chromosome_name
Residue_change.changed_residue
Residue_change.original_residue
Run.instrument
Run.run_date
Sequence.molecule
Sequence.sequence
Source.address
Source.email
Source.fax
Source.institution
Source.name
Source.tel
Study.abstract
Study.acknowledgements
Study.background
Study.conclusions
Study.key_results
Study.limitations
Study.objectives
Study.source_of_bias
Study.study_design
Study.study_power
Study.study_size_reason
Study.submission_date
Study.title
Study.update_date
Subject_descriptor.term
Subject_descriptor.vocabulary_name
Taxon.rank
Taxon.scientific_name
Transcription_change.changed_codon
Transcription_change.codon_position
Transcription_change.original_codon
Variation_assay.description
Web_resource.cost
Web_resource.estimated_size
Web_resource.url