docs: add docstrings for modules

GeoSpatialTools/distance_metrics.py

 """
+Distance Metrics
+----------------
 Functions for computing navigational information. Can be used to add
 navigational information to DataFrames.
 """

GeoSpatialTools/great_circle.py

-"""Class for a Great Circle object."""
+"""
+GreatCircle
+-----------
+Constructors and methods for interacting with GreatCircle objects, including
+comparisons between GreatCircle objects.
+"""
 
 import numpy as np
 

GeoSpatialTools/kdtree.py

 """
+KDTree
+------
 An implementation of KDTree using Haversine Distance for GeoSpatial analysis.
-Useful tool for quickly searching for nearest neighbours.
+Useful tool for quickly searching for nearest neighbours. The implementation is
+a K=2 or 2DTree as only 2 dimensions (longitude and latitude) are used.
+
+Haversine distances are used for comparisons, so that the spherical geometry
+of the earth is accounted for.
 """
 
 from . import Record

GeoSpatialTools/neighbours.py

-"""Functions for finding nearest neighbours using bisection."""
+"""
+Neighbours
+----------
+Functions for finding nearest neighbours using bisection.
+"""
 
 from numpy import argmin
 from bisect import bisect

GeoSpatialTools/octtree.py

+"""
+OctTree
+-------
+Constuctors for OctTree classes that can decrease the number of comparisons
+for detecting nearby records for example. This is an implementation that uses
+Haversine distances for comparisons between records for identification of
+neighbours.
+"""
+
 from dataclasses import dataclass
 from typing import List, Optional
 import datetime

GeoSpatialTools/quadtree.py

 """
+QuadTree
+--------
 Constuctors for QuadTree classes that can decrease the number of comparisons
-for detecting nearby records for example
+for detecting nearby records for example. This is an implementation that uses
+Haversine distances for comparisons between records for identification of
+neighbours.
 """
 
 from dataclasses import dataclass