Demo for level set functionality

[joewallwork]

It'd be great to include a demo on the level set functionality, as well as something in the docs.

[stephankramer]

Agreed, but note that this currently relies on a branch of petsc where I changed the interface to provide the additional information: https://gitlab.com/stephankramer/petsc/-/commits/skramer/levelset

I need to actually communicate with petsc-devs how they want this:

• add a (nullable) extra argument to DMAdaptMetric - the thing with that is that it then needs an implementation for DMAdaptMetric_[Mmg|ParMmg|Pragmatic] - I still need to figure out if and how parmmg supports this
• attach something to the DMMetric object - it's not really a metric thing, but then other non-metric adaptivity configuration options are also attached to it
  At the moment I've just made a new DMAdaptMetricLevelSet which only has a mmg implementation (2d or 3d but serial)

Then you also need an animate branch https://github.com/mesh-adaptation/animate/tree/levelset that provides the passes on the extra argument (both branches are probably fairly out-of-date now)....