RCPP - rewrite internal functions to use with examples without errors

[rspadim]

first functions to rewrite:

1. gravity
2. mode
3. mode_class

[rspadim]

associated error:

* checking examples ... [19s] ERROR
Running examples in 'NNS-Ex.R' failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: NNS_part_RCPP
> ### Title: NNS Partition Map - Internal Use
> ### Aliases: NNS_part_RCPP
> 
> ### ** Examples
> 
> set.seed(123)
> x <- rnorm(100) ; y <- rnorm(100)
> NNS_part_RCPP(x, y, type = NULL, order = NULL, obs_req = 8, min_obs_stop = TRUE, noise_reduction = "off", Voronoi = FALSE)
Error in NNS_part_RCPP(x, y, type = NULL, order = NULL, obs_req = 8, min_obs_stop = TRUE,  : 
  could not find function "gravity"
Execution halted