file system related structure elements can use it to have a variable
that can be accessed in the cfood that contains the file system path of
the structure element

Also, did some refactoring of the related StructureElements

MAINT: replace prints by logging calls

See merge request !97

F refactor default converters

See merge request !95

F subtree

See merge request !93

F date converter

See merge request !92

Release 0.3

See merge request !91

MAINT: Update .gitlab-ci.yml file to automatically build gitlab pages for the documentation

See merge request !89

Example cfood in the tutorials section

See merge request !90