Configuration & data are not in expected (XDG) locations

[khaeru]

Several related issues here that could be fixed together.

1. ixmp-config places a file at ~/.local/ixmp/config.json. This does not conform to the XDG base directory specification. The file should be at either:
   • ~/.local/share/ixmp/config.json — i.e. using $XDG_DATA_HOME or its default.
   • ~/.config/ixmp.json or ~/.config/ixmp/config.json — i.e. using $XDG_CONFIG_HOME or its default.
2. Likewise the localdb directory created with ixmp.Platform(dbtype='HSQLDB') is incorrectly at ~/.local/ixmp/localdb instead of ~/.local/share/ixmp/localdb.
3. ixmp.config.get() raises an exception if the configuration file is missing. Sensible defaults should be used instead.

[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.