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

if os.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:
        from IPython import get_ipython
        return get_ipython() is not None
    except ImportError:
        return False

Then replace:

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

This would be more robust.