1.0.0 alpha - Legend Switcher on WMS

[hugbe8]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines:https://github.com/infra-geo-ouverte/igo2/blob/master/.github/CONTRIBUTING.md#git-commit-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• New feature (Legend properties on wms-datasource 1.0.0-alpha.1 #309)

What is the current behavior? (You can also link to an open issue here)

1. Legend Style: The wmw apply only a legend style. It use always the default style.

2. Behavior of the legend display: On map tool, the legend is always displayed.

What is the new behavior?

1. Legend Style:
   The possibility of having several legends on a WMS. By a context file or by the catalog tool.

Origin:
options

optionsFromCapabilities: true

contexts

 legendOptions: {
          stylesAvailable: [{name: 'rain', title: 'Pluie'}, {name: 'raster', title: 'Défaut'}]
        },

By default:

• The legends come from GetCapabilities when it is available even if there are no stylesAvailable parameter.
• On using the stylesAvailable parameter, the first style of the list is applied.

Limitation: the multiple layers are not managing

2. Behavior of the legend display (Legend properties on wms-datasource 1.0.0-alpha.1 #309)
   The statut of the legend is conserver on the navigation between tools.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[X ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications:

Other information:

• the optionsFromCapabilities option was removed because it already obtains it by inheriting from datasource.interface.ts (packages/geo/src/lib/datasource/shared/datasources/wmts-datasource.interface.ts)
• the loop on the legends has been reversed to have the same behavior as OpenLayers which places the last layer of a list on top. (packages\geo\src\lib\layer\layer-legend\layer-legend.component.html)