[!278] Enable extendability of the visitor for custom UFL nodes

Merge branch 'feature/allow-custom-ufl-types' into 'master'

ref:dominic/dune-perftool We do want code bloat in dune-perftool for the
specific needs of a given application. We therefore allow custom UFL nodes to
procide their own visitor implementation. This was done previously for the
very specific case of a MathFunction node, but it can be done with any node.

See merge request [dominic/dune-perftool!278]

  [dominic/dune-perftool!278]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/278