improvements for NAD

[cole-brokamp]

• more clearly note large download file size to install NAD binary (ask user first?)
• vectorize nad() function to take more than one county and returned rbinded results
• better error messages for binding data frames together that have addr columns? shouldn't vctrs deal with this OK? (maybe S7 problem...)

[cole-brokamp]

keeping in mind that eventual future state will be cloud-library of pre-generated NAD files, or the ability to generally grab a pre-sliced NAD data.frame

[cole-brokamp]

There was a problem with [ methods for addr and addr_part S7 objects, which caused the unexpected errors when trying to bind_rows of a list of tibbles with addr or addr_street columns. I fixed this in the tiger-geocode branch, but will merge that to main soon so you can rely on using rbind or similar under the hood for the improvements to NAD