add variable to grid_mapping_atts

[dblodgett-usgs]

We now have:

nc_grid_mapping_atts(system.file("extdata/daymet_sample.nc", package = "ncmeta"))
# A tibble: 11 x 5
# id name                          variable                value     data_variable
# <dbl> <chr>                         <chr>                   <list>    <chr>        
#   1     0 grid_mapping_name             lambert_conformal_conic <chr [1]> prcp         
# 2     1 longitude_of_central_meridian lambert_conformal_conic <dbl [1]> prcp         
# 3     2 latitude_of_projection_origin lambert_conformal_conic <dbl [1]> prcp         
# 4     3 false_easting                 lambert_conformal_conic <dbl [1]> prcp         
# 5     4 false_northing                lambert_conformal_conic <dbl [1]> prcp         
# 6     5 standard_parallel             lambert_conformal_conic <dbl [2]> prcp         
# 7     6 semi_major_axis               lambert_conformal_conic <dbl [1]> prcp         
# 8     7 inverse_flattening            lambert_conformal_conic <dbl [1]> prcp         
# 9     8 longitude_of_prime_meridian   lambert_conformal_conic <dbl [1]> prcp         
# 10     9 _CoordinateTransformType      lambert_conformal_conic <chr [1]> prcp         
# 11    10 _CoordinateAxisTypes          lambert_conformal_conic <chr [1]> prcp