Fix no FINESSE_DIR set error
Traceback (most recent call last):
File "ligo_tune.py", line 3, in <module>
import pykat
File "/home/runner/pykat/pykat/__init__.py", line 51, in <module>
v = kat.finesse_version()
File "/home/runner/pykat/pykat/finesse.py", line 1884, in finesse_version
p = Popen([self._finesse_exec(), '-v'], stdout=PIPE)
File "/home/runner/pykat/pykat/finesse.py", line 1857, in _finesse_exec
self.__finesse_dir = os.environ.get('FINESSE_DIR').strip()
AttributeError: 'NoneType' object has no attribute 'strip'Should warn user better