Fix KeyError when _ environment variable is not set

[kangmg]

The _ environment variable is not guaranteed to exist in all environments. This PR replaces os.environ["_"] with os.environ.get("_", "") to prevent KeyError.

- if os.environ["_"].find("jupyter") == -1:+ if os.environ.get("_", "").find("jupyter") == -1:

PyXtal/pyxtal/XRD.py

Line 539 in f5560b4

ifos.environ["_"].find("jupyter") ==-1:

[kangmg]

The suggested fix is a minimal change to prevent the "KeyError", but a more robust approach would be to detect the notebook environment directly using IPython:

def_in_notebook():
try:
fromIPythonimportget_ipythonreturnget_ipython() isnotNoneexceptImportError:
returnFalse

Then replace:

- if os.environ["_"].find("jupyter") == -1:+ if not _in_notebook():

This would be more robust.