Refactor and add a TigerLine.get function that can do everything

[asinghvi17]

This removes the Format.jl dependency and makes life simpler for the end user, all they need to do is

TigerLine.get(2020, "county")

and it gives them a Shapefile.jl table that is ready for analysis.

This also factors the code that gets the list of files available under that heading out of the download function into a new function that is common.

get uses Scratch.jl to cache files so you don't have to keep re-downloading. This assumes that the source data is not going to change, which seems reasonable to me.

[TheCedarPrince]

@asinghvi17 , been tinkering a bit further with this and have one thought: I find the use of Scratch.jl clever but too clever. I wouldn't expect one of my mentees or students to know where to look to find their data after running get. What is your thought on using Scratch?

[asinghvi17]

Hmm, I wouldn't expect them to use the file path at all - the idea of get is that you set it and forget it, in some sense, and it caches the download for you.

If you want the file path you could always use download_tiger was my initial thought...

[TheCedarPrince]

That's fair @asinghvi17 -- I'll keep tinkering to show you how I am imagining things being used.