Replace hooks by copier features and improve updates

[dalito]

Changes:

• Replace cookiecutter-related hooks by standard copier features in copier.yaml
• Ask only the questions required, skipping superfluous ones.
• Prevent copier from changing files that already exist (see _skip_if_exists section in copier.yaml)
  • Note that even with this change applying the template to existing projects and updating causes too many changes. This is due to the design of the original linkml-project-cookiecutter which did not consider code-lifecycle-management as top priority.
• Add notes on migrating existing linkml-project-cookiecutter-based projects to this template.

Closes #4