Move from nosetests to pytest

[micheles]

Part of #144.
This was difficult because I discovered a lot of broken tests. Tests are broken for various reasons:

1. due to missing __init__.py files, several tests were not executes by nose, so they were not working anymore
2. some tests were running only on macos (there was a reference to a directory "Data" while the name is "data" and linux
   is a case-sensitive file system)
3. some tests were running only from specific directories; notice that using things like '../.../../data/the/file.csv'' is the worse
   thing you could do
4. there were syntax errors (like true instead of True and null instead of None in a file)
5. one test was depending on the environment variable GEMDATA which is not set
6. there was a mismatch rupid <-> rup_id
7. the way to extract the ruptures file in MFDFromSESMFDFromInputTest was wrong.

I have skipped 5 broken tests that should be addressed by the hazard team (search for unittest.SkipTest).

NB: I have no idea why files like openquake/mbt/tests/tools/fault_modeler/data/fault_model_01.test.xml changed!

[mmpagani]

All LGTM. Thanks a lot, MIchele. @cossatot you might want to take a look at the fault modeller tests.

[cossatot]

Great, thanks. I like pytest much more. I'll look into the fault modeler tests as well.