Generic interface for command-line app executors.

[j-woz]

From @GoogleCodeExporter on April 22, 2015 19:4

Provide a method to implement third-party command line executors.

Should be able to ship a third-party executor as a Swift include + Tcl package. 
 Constraints are:

* Must be able to dynamically load as Tcl package (current design can do this)
* May have declarations in user script/header script (TODO)
* Must not require changes to Turbine Tcl/C code to add new executor
* Must not require changes to STC to add new executor

I think the third-party executor would need:
- to implement the turbine_executor interface, and register itself with Turbine 
when the package is loaded
- declare its existence in the Swift include and specify a "run" tcl proc that 
should be called to launch the task

As a test to confirm it's working, we should be able to replace any 
special-cased coaster logic in STC and Turbine with generic logic.

Original issue reported on code.google.com by tim.g.ar...@gmail.com on 15 Aug 2014 at 9:21

Copied from original issue: j-woz/exm-issues#735