first pass get_text method for ALTO XML input

[rlskoeser]

update ALTO FileInput class get_text method to return lines of text in document order

• for each xml document in the zipfile:
  initialize as alto xmlobject, then
  • for each text block in the xml document:
    • for each textline in the text block, return string contents

Once basic text iteration is working, sort text by VPOS