Allow inspecting two variables at the same time

[forman]

The viewer should allow inspecting two variables at the same time. Here is possible workflow

1. selects a dataset, variable, time
2. pins (new!) the current selection
3. selects another dataset, variable, time
4. activates linked view (new!): viewer will spatially auto-align the two views.

Then there are multiple possibilities to show the two variables at once:

1. A slider will appear over or under the map, similar to here: Layer Swipe
2. A "spy hole" will appear centered under the mouse cursor, similar to here: Layer Spy

Note, with the linked views in place it is also already possible to adjust the opacity of the top-most layer.