Improved docstring for base analysis init.
Improved docstring for base analysis init.

Added a section that explains different ways to specify where to extract
the data from.

DiCarloLab-Delft/PycQED_py3,QudevETH/PycQED_py3,DiCarloLab-Delft/PycQED_py3,DiCarloLab-Delft/PycQED_py3