224 correct vignette visualization deployment

[PietrH]

The vignette: visualize-deployment-features.Rmd has gotten out of date, zero count observations are displayed by default as black multiplication symbols ×, not red circle markers.

After discussion with Damiano we've decided to update the vignette, not the function map_dep().

The function used to display red circle markers for zero count observations, but was updated to use an icon instead. This vignette was overlooked.

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Damiano noticed in this PR that an older issue is actually unsolved: #6 (comment) :

• map_dep() can differentiate between NA and 0
• map_dep() visualizes NA and 0 differently
  map_dep() offers users control over NA and 0 visualization:
• na_values_show
• na_values_icon_url
• na_values_icon_size
  Assertions were added for these last arguments.

[PietrH]

@damianooldoni Could you have a look at the rest of the vignette? Especially the mentions of the colours red or grey.

For example, is this still correct? I don't see any grey circles:

[damianooldoni]

Thansk @PietrH to ask me to review the vignette. It was time to update it. And the difference between NAs and 0 while visualizing the number of species was gone, apparently. I think I fixed it.
I would like to have your review. But I cannot point you as reviewer officially as you are the owner of this PR 🤣

[PietrH]

@damianooldoni, I've suggested some minor changes.

Mostly about test coverage, if you want we can split that off to a different pull request. In that last case I can contribute.

Do you think we should test the output of map_dep() for certain properties, such as what leaflet calls were used and so forth? Such a test might notify us of problems such as not plotting things we expect to be plotted, but is less strict than a snapshot.