Grants (or denies) this role access to a dimension.
access may be "all", "custom" or "none".
Note that a role is implicitly given access to a dimension when it
is given "ALL" acess to a cube.
If access is "custom", no access will be inherited
by the hierarchies of this dimension.
If the parent schema access is "ALL_DIMENSIONS",
this timension will inherit access "ALL".
See also the "all_dimensions" option of the "SchemaGrant" element.