[WIP] Refactoring to include outputs parsing

[gbrunin]

I have changed the structure of the code to better reflect the distinction between input and output parsers. Concerning the inputs, all the tests that were already there still pass, it's only a matter of adapting the namespace. For the outputs, I created objects such as PwOutput, that inherit from an abstract BaseOutput, that for now can be instantiated with a from_dir method (a from_files method could/should be added as well). A user with a job that ran in a given directory could get the outputs easily using this classmethod. In these from_dir methods, specific XML and/or standard output Parsers would be used to get the results. Each Parser would parse a single file, and the logic of parsing and extracting the outputs from the different codes would have to be implemented in each from_dir method. For instance, a NebOutput.from_dir would parse the standard output of the global computation and probably the standard outputs and/or XML files for each image. The extracted outputs would be stored as a simple dictionary and these objects would not rely on any external package. Then, in qe_tools.extractors, ASE and pymatgen objects could be constructed (e.g., ase.Atoms/pymatgen.Structure, band structures,...), allowing each to be optional dependencies.

This is only the base logic of the new structure and many things remain to be implemented. The idea now is to see what breaks with this in aiida-quantumespresso and how the parsing could be moved from there to here. Then, more will be added depending on the needs.