Use usmapdata package

[pdil]

The us_map implementation and associated mapping files have been extracted into a standalone package called usmapdata. This should greatly reduce the file size of usmap and prevent the NOTE caused by R CMD check.

This PR will track the removal of usmapdata related features including but not limited to:

• replacing us_map() implementation
• removing map generation scripts
• updating documentation to refer to usmapdata where needed
• standardizing documentation between packages so that information is not repeated unnecessarily (and to prevent future inconsistencies between documentation)

fixes #39