Replace hard coded project external paths with gda_root variable based paths.

This should allow jython unit tests to run without project specific project
external paths being set.

To set up the gda_root variable (needed only once per workspace):

  Window > Preferences > Pydev > Interpreter - Jython >
    String Substitution Variables tab > Add variable
      Key:   gda_root
      Value: /dls_sw/iXX/software/gda_versions/gda_X.YY/

Project paths based on gda_root assume that a standard deployment is in
use, with the workspace and a workspace_git subdirectory immediately
under the specified gda_root directory.