Error: sEPSG not valid

Hello,

On line 177 of `btb_smooth.R`, you check that the length of the `sEPSG` argument is equal to 4 characters when the function is provided with a `sf` object:

```r
stopifnot("sEPSG not valid "= identical(nchar(as.character(sEPSG)),4L))
```

However, the epsg code could be of a length greater than 4 character. You have such an instance in the second example that you provide:

```r
# Example 2
data(reunion)
# Call mode 1: classic smoothing - automatic grid
reunionSmoothed <- btb_smooth( pts = reunion,
sEPSG = "32740",
iCellSize = 200L,
iBandwidth = 400L)
```

`reunion` is a data.frame, but if you transform it to a `sf` object and try to run the code, it errors:

```r
# Example 2
data(reunion)
reunion <- sf::st_as_sf(reunion, coords = c("x","y"), crs = 32740)
# Call mode 1: classic smoothing - automatic grid
reunionSmoothed <- btb_smooth( pts = reunion,
iCellSize = 200L,
iBandwidth = 400L)
Error in btb_smooth(pts = reunion, iCellSize = 200L, iBandwidth = 400L) :
sEPSG not valid
```

Is there a reason I am not understanding for the check for a epsg length equal to 4?

Thank you, and thank you for your very useful package!