Expose ctkDICOMRetrieve class and its methods to be invokable from Python
Add string property to CLI .xslt to retrieve Qt text field
Update doxygen table and string maps to vectorValueProperty.

For issue #306.
Trim CLI string parameter values, so that if empty, the parameter name is not passed to backend
CLI pass explicit empty string

For issue #307.
Removed unnecessary #include <iostream>

For issue #307.
Don't substitute command line arg with default

For issue #307, as user may have explicitly removed the default.
Merge branch '306-cli-string-not-passed-to-backend'

Closes #306.
Merge branch '307-cli-empty-string-not-rejected'

Closes #307.
Don't trim string arguments to CLI.

For #307.
Merge branch '307-cli-empty-string-not-rejected'

Re-closes #307.
Pass all string values as they are. Qt quotes them already. Issue #307.