Modify logging message from critical to warning.

[Nush395]

When imas_core is not available a critical level log is output

IMAS-Python/imas/backends/imas_core/imas_interface.py

Line 34 in 7f2e15f

logger.critical(

Would it possible to change this to a logging.warning instead?

Context: When we import IMAS-Python in TORAX we log this message and as it appears as a critical log a few users have mistaken it for an error.

[Nush395]

thank you!