gh_covering should be S3

Currently an error:

```
GH6 = gh_covering(SG$poly, minimal = TRUE)
```

```
Error in gh_covering(SG$poly, minimal = TRUE) : 
  Object to cover must be Spatial (or subclass)
```

This should just dispatch:

```
GH6 = gh_covering(as_Spatial(SG$poly), minimal = TRUE)
```