Remove the unused FlattenSequences class from ir.postprocessing
Add a test case for vector extraction
Treat explicitly and implicitly modelled scalar fields the same in map_field
Concerning Scalar Fields
pystencils permits users to omit the trivial index shape (1,) for scalar fields.
We distinguish D+1-dimensional explicit scalar fields, where the trivial D+1st dimension is modelled,
and equivalent D-dimensional implicit scalar fields.
To make sure that explicit scalar fields can always be mapped onto D-dimensional data structures, we ignore their trivial index dimension.
