Refactor to consolidate some common Amuse reader functionality

[thisisparker]

What started as a pretty targeted rewrite turned into something of a major refactor because as I was touching various parts of the script it made sense to make them all a little better. Notably, this PR should close:

• Some code repitition in Amuse readers for finding latest puzzles #10, which was the original point
• LA Times puzzles now primarily available in Amuse reader #8, which became kind of a freebie
• Better filenames #13, which made it easier to fix the preceding two

Hopefully the classes are a little easier to follow now, too!