Add WMS layer support

[amotl]

Regarding integration of WMS layers, we did some initial implementation on top of Map Panel but unfortunately lost the code.

The reference documentation for vanilla Leaflet is Using WMS and TMS services within Leaflet.

When coming back to this, we might want to look at integrating features from the Hypermap-Registry and the OSM Layer Index.

• https://cga-harvard.github.io/Hypermap-Registry/
• https://github.com/cga-harvard/Hypermap-Registry
• https://github.com/osmlab/editor-layer-index

[amotl]

@as17 requested an option to choose the various openstreetmap layers: topology (https://opentopomap.org), satellite, etc. within grafana#242.

[amotl]

Hi,

I just submitted #125, which are some rough notes including code snippets based on leaflet.wms. However, I believe the GPA-PhasorMap already has a sweet and complete WMS implementation, see https://gridprotectionalliance.github.io/GPA-PhasorMap/Settings.html.

With kind regards,
Andreas.