fix: sectioner dissociated titles from their chunk (#1861)
### disassociated-titles
**Executive Summary**. Section titles are often combined with the prior
section and then missing from the section they belong to.
_Chunk combination_ is a behavior in which two succesive small chunks
are combined into a single chunk that better fills the chunk window.
Chunking can be and by default is configured to combine sequential small
chunks that will together fit within the full chunk window (default 500
chars).
Combination is only valid for "whole" chunks. The current implementation
attempts to combine at the element level (in the sectioner), meaning a
small initial element (such as a `Title`) is combined with the prior
section without considering the remaining length of the section that
title belongs to. This frequently causes a title element to be removed
from the chunk it belongs to and added to the prior, otherwise
unrelated, chunk.
Example:
```python
elements: List[Element] = [
Title("Lorem Ipsum"), # 11
Text("Lorem ipsum dolor sit amet consectetur adipiscing elit."), # 55
Title("Rhoncus"), # 7
Text("In rhoncus ipsum sed lectus porta volutpat. Ut fermentum."), # 57
]
chunks = chunk_by_title(elements, max_characters=80, combine_text_under_n_chars=80)
# -- want --------------------
CompositeElement('Lorem Ipsum\n\nLorem ipsum dolor sit amet consectetur adipiscing elit.')
CompositeElement('Rhoncus\n\nIn rhoncus ipsum sed lectus porta volutpat. Ut fermentum.')
# -- got ---------------------
CompositeElement('Lorem Ipsum\n\nLorem ipsum dolor sit amet consectetur adipiscing elit.\n\nRhoncus')
CompositeElement('In rhoncus ipsum sed lectus porta volutpat. Ut fermentum.')
```
**Technical Summary.** Combination cannot be effectively performed at
the element level, at least not without complicating things with
arbitrary look-ahead into future elements. Much more straightforward is
to combine sections once they have been formed from the element stream.
**Fix.** Introduce an intermediate stream processor that accepts a
stream of sections and emits a stream of sometimes-combined sections.
The solution implemented in this PR builds upon introducing `_Section`
objects to replace the `List[Element]` primitive used previously:
- `_TextSection` gets the `.combine()` method and `.text_length`
property which allows a combining client to produce a combined section
(only text-sections are ever combined).
- `_SectionCombiner` is introduced to encapsulate the logic of
combination, acting as a "filter", accepting a stream of sections and
emitting the same type, just with some resulting from two or more
combined input sections: `(Iterable[_Section]) -> Iterator[_Section]`.
- `_TextSectionAccumulator` is a helper to `_SectionCombiner` that takes
responsibility for repeatedly accumulating sections, characterizing
their length and doing the actual combining (calling
`_Section.combine(other_section)`) when instructed. Very similar in
concept to `_TextSectionBuilder`, just at the section level instead of
element level.
- Remove attempts to combine sections at the element level from
`_split_elements_by_title_and_table()` and install `_SectionCombiner` as
filter between sectioner and chunker.
