Replace is_many with ARRAY type properties

1) performance gain (minor)
2) because we want to add some functionality to add_operation.
Add ability to declare constants on Operations

Constants cannot be the target of additional links, they automatically
connect to the 'input connector'.  Constants propagate when the
operation is added to a dag via 'add_operation'.
Automatically add constant_values to execute

They can be overridden since @_ comes after them in %inputs.
Merge pull request #240 from davidlmorton/wb_constants

Add ability to declare constants to WorkflowBuilder Operations