List entities example

[Simard302]

Overview

This PR creates a new example for inspecting a new structure and listing many of its entities and their properties.

Changes made

• Created inspect example "list_entities"
  • Count the number of waters
  • Retrieve the names and residue codes for all covalent ligands
  • Count the number of AltLoc positions
  • Retrieve the names and chains of all polymer entities

How to test

Ensure the names and counts of all the properties are correct for multiple PDBs

• By default, uses 1PTH (which has many of these)
• Can edit the PDB in the code

Specific review requests

For each of the examples:

• Are there enough comments to sufficiently describe the process?
• Are the comments accurately describing the process?
• Are there any anti-patterns in the examples?
• Can the examples be more efficient?