renamefields: Rename some field names in a struct

Changes struct field names from cell array list of old names to
names in list of new names. Optional fourth argument suppresses
error message if struct does not contain an old name.
Examples
struct2=renamefields(struct1,{list old names},{list new names})
Changes the names given in "list old names" to those given
in "list new names"
struct2=renamefields(struct1,{list old names},{list new names},true)
Changes those names from old names, if present in struct1 to names
from new names
Can also swap names e.g.
struct2=renamefields(struct('a',1,'b',2,'c',3),{'a','b'},{'b','a'})
See also selectfields
See doc renamefields