[IMP-dev] More proposed simplifications to IMP

I'd like to remove support for explicitly listing headers/source files in SConscript files. The default behavior, and the one we recommend that people use is that the SConscript simply finds all the .cpp or .h files in the appropriate directories (like how it finds all tests in the test directory). Since broken .cpp or .h files mixed in with the real source make reading/greping the code more confusing and they can easily be moved elsewhere there isn't really much point in the excluding files by not listing them. And it creates the prospect for bugs when headers or source files are left out of the list. Let me know if there are any objections.