Configuration & data are not in expected (XDG) locations about ixmp

[khaeru]

An example of item 3: on #128 @danielhuppmann wrote:

in the current setup, the local default HSQLDB is touched every time a user re-installs and runs the tests on the tutorial notebooks. So the default database files become quite bloated over time. Would it be possible to connect the tutorial-notebook tests to a temporary db?

This could be fixed by the following new functionality:
—check for an environment variable, IXMP_CONFIG.
—look in that directory for config.json, before any other path such as the current defaults.

Then the tests could pass a specific value of IXMP_CONFIG in order to not modify the user's configuration.