[Issue #42] OS-dependent path in tools/ioparser.py

[Didou09]

Motivations:

• As pointed out in issue #42, when @ruairidhcumming tied to load exiobase2, an error message was raised saying that the population.txt file was not found
• @konstantinstadler suggested that it may have to do with the hard-coded relative path at line 656 of tools/ioparser.py

Main changes:

• fixes OS-specific hard-coded relative path in parse_exiobase2() for population.txt, as pointed out above

Issues:

• Fixes only partially the problem brought up by issue #42, by making the relative path OS-independent using os.path.join()
• However, it can be seen that:
  • the problem is solved when pymrio is installed from a github clone
  • the problem is not solved when installed from pypi using pip The reason for this is that the pip installation does not seem to include the following diretories and data: exio20

Indeed, when checking the content from my github cloned repo:

(base) ls /home/didier/Documents/Projects/pymrio/pymrio/mrio_models/
total 16K
drwxrwxr-x 4 didier didier 4,0K oct.  30 15:01 .
drwxrwxr-x 7 didier didier 4,0K oct.  30 16:13 ..
drwxrwxr-x 4 didier didier 4,0K oct.  30 15:01 exio20
-rw-rw-r-- 1 didier didier    0 oct.  30 15:01 __init__.py
drwxrwxr-x 3 didier didier 4,0K oct.  30 15:01 test_mrio

When checking from the pip-installed version:

(base) ls /home/didier/anaconda3/lib/python3.7/site-packages/pymrio/mrio_models/
total 16K
drwxrwxr-x 4 didier didier 4,0K oct.  30 14:48 .
drwxrwxr-x 6 didier didier 4,0K oct.  30 14:48 ..
-rw-rw-r-- 1 didier didier    0 oct.  30 14:48 __init__.py
drwxrwxr-x 2 didier didier 4,0K oct.  30 14:48 __pycache__
drwxrwxr-x 4 didier didier 4,0K oct.  30 14:48 test_mrio

Clearly, exio20 is not included in the pip installation, which means users using pip will have the same issue even after this PR. Opening a separate issue to deal with this, which should be done both in the setup.py and in a MANIFEST.in from what I know.