secured most of the list_pointer accesses

generalized Plugin(Extract) so that we can also create vector views

better docs

quick generalization of Plugin(Extract): we can now extract arbitrary
combinations of the field's components

small tweaks

missing return in case of error

Added automatic documentation generation for plugins

- better file names for views created by plugins
- use the file name as a guess for view_save_{ascii,binary}

forgot to remove debug printf

typo

New plugin "Plugin(Extract)" to extract the i-th component of a field