Python 3 syntax errors in lalapps/src/string

lalapps/src/string/cosmicstring.py and lalapps/src/string/lalapps_cosmicstring_pipe.py are both incompatible with python3 due to inconsistent use of tabs and spaces:

$ python -m py_compile lalapps/src/string/*.py
Sorry: TabError: inconsistent use of tabs and spaces in indentation (cosmicstring.py, line 309)
Sorry: TabError: inconsistent use of tabs and spaces in indentation (lalapps_cosmicstring_pipe.py, line 218)

This will need to be corrected for the next release to support python>=3.