Define variable as a contour lines

[brosaplanella]

Description

In some cases (especially when moving fronts involved) we want to define a variable as a contour line of another variable. One example is for electrolyte depletion: if we want to track the depletion front we need to define it as the set where the electrolyte concentration is equal to zero (or first becomes zero).

By adding this, we could easily plot these variables with the existing functionality.

Motivation

Some problems where this is relevant:

• Tracking electrolyte depletion front.
• Phase-change (as in melting) problems.
• LFP batteries show a lithiation front (not sure how this is captured in the model though).

(feel free to add other cases if you can think off)

Possible Implementation

I don't know how to start with this, so looking for some discussion on how to tackle it.

Additional context

No response

[kratman]

@brosaplanella Can we close this since nothing has really happened for it