Set Matplotlib cache directory to writable directory

Matplotlib created a temporary config/cache directory at /tmp/matplotlib-tufz791h because the default path (/local/home/lstanalyzer/.config/matplotlib) is not a writable directory; it is highly recommended to set the MPLCONFIGDIR environment variable to a writable directory, in particular to speed up the import of Matplotlib and to better support multiprocessing.

Maybe we could change this directory