ExecutableNode Process support

[johnhaddon]

This does the following :

• Adds an API to TaskPlug to provide saner public access to the ExecutableNode methods, which are now "protected" (documented as protected but not actually protected yet for backwards compatibility reasons).
• Exposes the processing performed by ExecutableNodes via the Process class. This makes it possible to monitor them.
• Makes automatic string substitution work for ExecutableNodes (Support automatic variable substitution during ExecutableNode::execute() #887).
• Removes the manual string substitutions from the various nodes which had bothered to implement them.

I think it's important that we do this, as it puts the ExecutableNode on a much better footing alongside the rest of our APIs, and #887 is the cause of numerous bugs (some of them yet to be discovered). It'd be great if we could go a bit further and actually break compatibility to get things really tidy, but I haven't done that here since we were hoping for 0.23 to remain the major version for a while.

It's important to note that although substitutions are now automagic, that is only the case if you use the TaskPlug API instead of the deprecated "protected" API to execute nodes (since I removed the manual substitutions). I've updated all Gaffer's code so this is the case, but there may be other code calling ExecutableNode::execute() directly rather than via a dispatcher (in Caribou perhaps?). If we're concerned about that, then I can split the removal of manual substitutions out of this PR, and put it into one we merge at a later date.

[andrewkaufman]

Seems like an odd test to have failed on gcc 5 only

======================================================================

ERROR: testSerialisation (GafferUITest.EditorWidgetTest.EditorWidgetTest)

----------------------------------------------------------------------

Traceback (most recent call last):

File "/home/travis/build/ImageEngine/gaffer/install/gaffer-0.23.1.0-linux/python/GafferUITest/TestCase.py", line 58, in tearDown

self.assertEqual( widgetInstances, [] )

AssertionError: Lists differ: [<GafferUI.TabbedContainer.Tab... != []

First list contains 6 additional elements.

First extra element 0:

<GafferUI.TabbedContainer.TabbedContainer object at 0x327aed0>

+ []

- [<GafferUI.TabbedContainer.TabbedContainer object at 0x327aed0>,

- <GafferUI.BusyWidget.BusyWidget object at 0x32830d0>,

- <GafferUI.Widget.Widget object at 0x327af50>,

- <GafferUI.InfoPathPreview.InfoPathPreview object at 0x327ac10>,

- <GafferUI.PathListingWidget.PathListingWidget object at 0x327ac50>,

- <GafferUI.Spacer.Spacer object at 0x3283110>]

[andrewkaufman]

So I guess its now possible to monitor an executable, but we haven't exposed any monitors that do so? What did you have in mind for such a monitor?

[johnhaddon]

I don't have any particular use cases for executable monitoring (maybe dispatcher debugging?), but I think as a general principle we should allow monitoring of any type of node process. Really, the main benefit right now is having the automatic substitutions...

[johnhaddon]

I've split out the removal of the manual string substitutions into #1674, so that can be merged for a future major version, and this can be merged now.