Execute tutorial notebooks during install-and-test in temporary db

[danielhuppmann]

Currently, when installing and running unit tests, the tutorial notebooks are executed using the local default database. Over time, this db will become clogged by thousands of transport problems. It would be better to use temporary database instead.

This could be achieved by replacing the line
"mp = ix.Platform(dbtype='HSQLDB')"
in any notebook by:
"mp = ix.Platform(dbprops=os.path.expanduser(os.path.join('~', '.local', 'ixmp', 'unittest')), dbtype='HSQLDB')".

Where all files starting with unittest in ~\.local\ixmp will be deleted after the last unit test.

Note that changing this in the notebook itself would defeat the purpose...

[khaeru]

This was fixed in #130.