Event selection and build_boundary integration

[rwilson]

Currently the EventSelection? tool generates a python fragment containing MUX filenames and the global weight, in the hope that the generated module would be used in build_boundary.py.

A better approach would be to modify build_boundary.py to get urs2sts() to take the generated older generated event_<event#>.list file which already contains the MUX filenames and weight. The approach would be to remove the existing 'urs_filenames' and 'weight' parameters and replace them with a full path to the *.list file plus the 'base' path to the MUX files.

Changes to build_boundary.py should be made with a view to eventually including the code into ANUGA proper, which means removing any reference to project-specific values and passing them in as a parameter.

Note that any code existing outside ANUGA may hacve references to Numeric that must be changed when ANUGA goes to numpy.