Improve variable naming in expressive code section

[willingc]

This PR makes several changes:

• Reorders rules to put meaningful names before short names (it's a common mistake for people to opt for brevity over clarity)
• Expand precip to precipitation as it has less cognitive load and is more readable
• Have directory and file names be either _ or -. Personally, I would use _ unless I was working with web/JS code and Python

[sneakers-the-rat]

+1 on this and it would probably be worth clarifying that python module names must use _ because names with - are not valid names

I will often use - in my package name to differentiate it from the package module name (and also that's javascript brain leaking in) but that might be too much subtlety in convention for newbies :)

[lwasser]

Friends - i opened #44 I do think that using verbs for function names is really important. BUT I also have observed in a complex workshop that is having a really long functions and variable names while expressive can make the code harder to read (that might be just my opinion), but I wanted to discuss it. I'm fine with merging this as is, especially given you both seem to agree that more verbose is better. I suppose for me I struggle with really long names!

[lwasser]

@sneakers-the-rat @willingc so here what i LOVE and like to suggest is

• use a verb for function names

What i love less is -- if we write everything out now you have

convert_fahrenheit_to_kelvin(temperature_fahrenheit)

and as you add more of these and typing and such the line lengths get really long. At some point the code becomes longer and more to read. so i'm really torn about some of these suggestions but also open to merging and discussing separately!