Add 'parent' and 'children' properties to annotations and allow all tiers types with 'filter'

[coxchristopher]

This pull request proposes to add two new properties to Annotation objects:

1. parent, which provides a reference to the Annotation object that represents the parent of this Annotation (if one exists; None if this is a root tier); and
2. children, which provides a list of Annotation objects that are the children of this Annotation (if any exist; empty if this Annotation has no children).

These properties are meant to make it easier to traverse annotations in ELAN transcripts hierarchically by following parent and child relationships between annotations on different tiers. It is already possible to navigate from child tiers to parent tiers with Tier objects; this pull requests adds similar functionality to Annotation objects, as well.

This pull request also fixes a few minor typos (in comments, e.g., "Symbolic-Associtation" -> "Symbolic-Association") and adds the option of filtering non-time-aligned annotations using the starting and ending timeslots of their nearest time-aligned ancestor annotation, making it possible to use the filter method on any tier, regardless of its stereotype.