use library(sf) in vignette

[romainfrancois]

As part of testing reserve dependencies of the upcoming dplyr 1.0.8 (released soon), we've identified a potential issue with the vignette of this package. The tests give:

# salem

<details>

* Version: 0.2.0
* GitHub: NA
* Source code: https://github.com/cran/salem
* Date/Publication: 2020-11-05 16:40:05 UTC
* Number of recursive dependencies: 62

Run `cloud_details(, "salem")` for more info

</details>

## Newly broken

*   checking re-building of vignette outputs ... WARNING
    ```
    Error(s) in re-building vignettes:
      ...
    --- re-building ‘introduction.Rmd’ using rmarkdown
    --- finished re-building ‘introduction.Rmd’
    
    --- re-building ‘recreating_analyses.Rmd’ using rmarkdown
    Joining, by = "Month"
    Quitting from lines 348-367 (recreating_analyses.Rmd) 
    Error: processing vignette 'recreating_analyses.Rmd' failed with diagnostics:
    Can't convert NULL to a symbol.
    --- failed re-building ‘recreating_analyses.Rmd’
    
    SUMMARY: processing the following file failed:
      ‘recreating_analyses.Rmd’
    
    Error: Vignette re-building failed.
    Execution halted
    ```

Loading sf with library(sf) seems to be fixing it. I suppose this is something about method registration that is not done the same way with requireNamespace()