create_ring: create a ring function
In mengluchu/APMtools: statistical air pollution mapping tools

create a ring function

create_ring(
  inde_var,
  normalize = F,
  buffers_in = c(0, 25, 50, 100, 300, 500, 800, 1000, 3000),
  buffers_out = c(25, 50, 100, 300, 500, 800, 1000, 3000, 5000),
  number_roadtypes,
  Road_varname = "ROAD"
)

`inde_var`	a dataframe with ROAD variables named 'ROAD', n types of roads, now it is not a very generic function, need the data to be formated in a way as the buffers are ordered (e.g. 25m, 50m, 100m)
`normalize`	if T, devide the ring by the area of ring. Default is faulse
`buffers_in`	buffer sizes of the inner rings
`Road_varname`	the name of the road variable, grepl styple e.g. 'ROAD'
`buffer_out`	buufer sizes of the outter rings