clean_dup: Function to clean duplicated longitude and latitude data
In luismurao/hdm: hdm: "Habitat distribution models"

Clean duplicated longitude and latitude data using threshold distance which is a distance (in grades) between points to be considered duplicates.

1	clean_dup(data, longitude, latitude, threshold = 0)

`data`	A data.frame with longitude and latitude data
`longitude`	A character vector of the column name of longitude.
`latitude`	A character vector of the column name of latitude.
`threshold`	A numerc value representig the distance (in grades) between coordinates to be considered as a duplicate.