change the path of the datasets and datasets-config to take into acco…

[chrplr]

…unt XDG_DATA_HOME, and use .local/share otherwise

I propose to use .local/share or $XDG_DATA_HOME (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html) as a base directory for both datasets and datasets-config.

But I am not sure if this will will not break with the install script (!), unless it relies on constant.py