Updates of the Section 5: 'Vocabulary Enhancements'

[mservillat]

This PR updates the section 5. A new sub-structure is proposed:

• 5.1: Evolution of the Data Product Type vocabulary
  • With the subsubsections: Event list, Response functions, Event bundle, Advanced data products, Summary table, Clarification of ``flux'' in some term definitions
• 5.2: DataLink vocabularies
• 5.3: UCD Enhancements
  • With the subsubsections: Pulse Height, Electromagnetic spectrum description in UCD, Event Type, Particles, Statistical UCDs, Evolution of UCD list
• 5.4: MIME-type Enhancements