Change examples to use location instead of fips`

[sglyon]

This is related to #14 and #15

We made a couple changes to the backend to prepare for international data.

The main change is that we renamed the fips column location for some of the views (including demographics and economics)

@TomGoBravo we would like to make the same change for the covid endpoint, but wanted to give you a chance to update your code first. The upgrade path to the new api is as follows:

• Replace usage covid(fips=XX) endpoint with covid_us(location=XX)
• Either use location in your code that handles our data or rename the column to fips
• Note that the covid_us endpoint will not return a vintage, solving the issue in query for newest vintage of each variable  #2