output of as_flights_package() should pass R CMD check

[Simon P. Couch]

The package in the unit tests currently generates the following with devtools::check():

Duration: 11.7s

> checking Rd cross-references ... WARNING
  Missing link or links in documentation object 'flights.Rd':
    ‘get_airlines’ ‘get_airports’
  
  See section 'Cross-references' in the 'Writing R Extensions' manual.

> checking for code/documentation mismatches ... WARNING
  Data codoc mismatches from documentation object 'weather':
  Variables in data frame 'weather'
    Code: day dewp hour humid month origin precip pressure temp time_hour
          visib wind_dir wind_gust wind_speed year
    Docs: humid origin precip pressure temp, dewp time_hour visib
          wind_dir, wind_speed, wind_gust year, month, day, hour

As well as these messages:

Warning: The existing 'airlines.Rd' file was not generated by roxygen2, and will not be overwritten.
Warning: The existing 'airports.Rd' file was not generated by roxygen2, and will not be overwritten.
Warning: The existing 'flights.Rd' file was not generated by roxygen2, and will not be overwritten.
Warning: The existing 'planes.Rd' file was not generated by roxygen2, and will not be overwritten.
Warning: The existing 'weather.Rd' file was not generated by roxygen2, and will not be overwritten.

Ideally, the .Rd files should be generated from roxygen2 documentation in .R files rather than the other way around. This might fix the WARNINGs as a side effect.🐧