* Upgrade to python3.7 and pandas 0.3.4 because pandas has dropped support for
python 3.4 -> There are some API changes in pandas, and I've update the code
accordingly.
* Set pytest as the default test runner
The web server is included as a submodule.
The dependencies for the web (tornado) are not installed by default, but they
can be installed as an extra:
```
pip install soil[web]
```
Once installed, the soil web can be used like this:
```
soil-web
OR
python -m soil.web
```
There are other minor changes:
* History re-connects to the sqlite database if it is used from a different
thread.
* Environment accepts additional parameters (so it can run simulations with
`visualization_params` or any other in the future).
* The simulator class is no longer necessary
* Logging is done in the same thread, and the simulation is run in a separate
one. This had to be done because it was creating some problems with tornado not
being able to find the current thread during logs, which caused hundreds of
repeated lines in the web "console".
* The player is slightly modified in this version. I noticed that when the
visualization was playing, if you clicked somewhere it would change for a
second, and then go back to the previous place. The code for the playback
seemed too complex, especially speed control, so I rewrote some parts. I
might've introduced new bugs.
