Add testing functions using pytest

[BenjMy]

Good practices in soft dev consist in implementing testing functions.

As for now, the wrapper is limited to exploratory testing. To have a complete set of manual tests, here is a tentative list of the pyCATHY features:

• pycathy.tools
• [ ] test that the source files from the bitbucket are correctly fetched
• pycathy.DA.cathy_DA
• [ ] test that the DA returns an updated state
• pycathy.ERT.simulateERT
• [ ] test that the dataframe contains only positive values of apparent resistivities
• pycathy.plotters.cathy_plots
• [ ] test plots features (colorbar, export, ...)

These would be done automatically using pytest.

• [ ] Need to update the makefile with test run
• [ ] Need to update the requirements.txt with pytest

Milestone set to v.0.0.2