reference
When 
reference is 
cell, the direction is crystallographic,
its parent must be a cell and the direction indices must be indicated.
When 
reference is 
atoms, the direction is atomic and 
must contain two non coincident atoms defining a line.
Example: <direction ... reference="cell"/> (default)
Allowed values: cell, atoms (optional)
u, v, w
Set the indices 
u, v, w defining a crystallographic direction.
The three indices cannot be all zero. An error is shown when these 
indices are not indicated for a crystallographic direction or when 
they are indicated for an atomic direction. 
Example: <direction ... u="1" v="0" w="0"/> (no default)
Allowed values: integer (required, coupled)
vectors
Set the type of cell vectors, 
conventional or 
primitive,
used to determine the direction with 
u, 
v, 
w indices.
When the lattice is primitive there is no difference between
conventional or 
primitive cell vectors.
An error is shown when 
vectors is set for an atomic direction.
Example: <direction ... vectors="conventional"/> (default)
(defined in GAMGI_PHYS_DIRECTION_VECTORS)
Allowed values: conventional, primitive (optional)