* 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
Now the environment does not deal with history directly, it delegates it to a
specific class. The analysis also uses history instances instead of either
using the database directly or creating a proxy environment.
This should make it easier to change the implementation in the future.
In fact, the change was motivated by the large size of the csv files in previous
versions. This new implementation only stores results in deltas, and it fills
any necessary values when needed.
