Fix bug related to PYTHONPATH setting
Fix bug related to PYTHONPATH setting

Fix a bug that would incorrectly set the PYTHONPATH in setenv.py.  The script
would only respect what PYTHONPATH was set to at the time of running setenv.py
rather than always appending the correct Clawpack python path.

clawpack/clawpack-4.x,clawpack/clawpack-4.x,clawpack/clawpack-4.x,clawpack/clawpack-4.x