Generalize types used for visualization

This came up in https://github.com/trixi-framework/Trixi.jl/pull/907#discussion_r729051487: It would be nice to generalize data types used for visualization methods. For example, it would be awesome to be able to plot something like measurements on slices, similar to https://trixi-framework.github.io/Trixi.jl/stable/differentiable_programming/#Propagating-errors-using-Measurements.jl. This basically requires replacing hard-coded `Float64` by the element types of `u` and `x` etc.

This is in the same spirit as #910

Start with the visualization methods discussed in PR #907 and compare their hard-coded Float64 usage with the Measurements.jl error-propagation example in the documentation. Trace the types of u and x through the affected methods, then verify that measurements on slices can be plotted without losing their element types.

Written by the indexing model from the issue text.