Additional features reducing user boilerplate code #26

Thanks for this initiative.

Is it the right place to add high level functions to reduce boilerplate code for some user applications ?

For instance, here are the following features that I find painful to do and would be nice to have in a package:

• marking entities (codim 0 or 1 essentially) based on geometric locator functions, see https://bleyerj.github.io/comet-fenicsx/tips/mark_facets/mark_facets.html
• creating cell-wise constant fields based on a list of values and associated cell markers, see https://bleyerj.github.io/comet-fenicsx/tips/piecewise_constant_field/piecewise_constant_field.html
• generic eigensolver interface
• Newton solver with user callback
• setting up multiple DirichletBC, especially when dealing with mixed functionspaces or subspaces and time-dependent bcs, see for instance here https://github.com/MEALORII/TP_MEALOR/blob/main/mealor/mealor/boundary_conditions.py (intended for elasticity)
• evaluation of a field on a set of points

Tell me what kind of feature would fit in this project, I would be happy to contribute