Entirely restructured Xflow concerning ChannelNodes
Implemented new syntax to use Prototypes/dataflows via compute.

Use compute="dataflow['dataflowUrl']".
Can be used with filters, too:
compute="result = dataflow['dataflowUrl'](a,b,c,d,e,f)"
Added test cases for empty/missing arguments in Xflow.
Fixed xflow tests.
Implemented DataAdapter for new <dataflow> and <compute> node. Untested.