Use bulk CreateElements endpoint on PageXmlParser

We can change the implementation of PageXmlParser.save method:

• build an element (as a dict) for all page text_region, and reference them with the XML region:

regions = [
  (region, {"type": "paragraph", ...})
  for region in self.pagexml_page.page.text_regions
]
}

• create all relevant elements through CreateElements and store their ID in relation to the region (so we can find later on the paragraph created per region)
• list all paragraph transcriptions for ALL regions, and store them in a generic list (with the element id)
• iterate over all lines in a paragraph and build a list of elements to create through a single CreateElements call per paragraph
• list all line transcriptions for ALL line, and store them in a generic list (with the element id)
• create all transcriptions at the end of exec (for the page), by calling CreateTranscriptions
• remove the save_element_transcriptions and save_element methods