IterationSpaces compare equal if defined on the same Set
Args compare equal if defined on same data, map, index, access descriptor
Document DataSet equality operator
Minor refactoring of Dat tests
Plan: check for arg object identity when computing staging info
Dats compare equal if defined on the same DataSet and data
data argument is optional in device.Global
Globals compare equal when having the same dim and data
Maps compare equal if defined on the same iterset, toset, arity, data
Merge pull request #196 from OP2/equality

Equality operators

Implement equality operators for IterationSpace, Arg,
DataSet, Dat, Global, Map and unit tests for them.