If the env var XDG_CACHE_HOME is set, the cache directory is
"$XDG_CACHE_HOME/ianardap", else if HOME is set, it is
"$HOME/.ianardapcaches", else no cache is used but a loud
warning is displayed at the end of the result to encourage
users to set a cache directory.
Adapted pytest tests, no change needed on test_exe_matrix. They are
obviously slower with no cache. NB: two tests now fail independently
of this change.