Adds screening features to the API library

[Peteracn96]

New release of the code with screening functionalities added and tested.

New ScreeningConfiguration class for parsing the new screening configuration input file. An example of such was added to the new folder screeningconfig contained in the folder examples. A new CRYSTAL model for MoS2 was added.

Most new methods implemented in ExcitonTB.cpp, containing the screening calculations.
Lattice.cpp has new method to generate reciprocal lattice by a G-vector cutoff.

Two new scripts, write_screening.cpp and read_screening.cpp, usable out of the box, giving the user straightforward access to the two main new functionalities:
(i) computing the numerical RPA static dielectric function;
(ii) computing the exciton using the RPA screened potential.

The user can use the .dat file containing a list of momentum points inside the new data folder and the script write_screening.cpp to generate the curves of the new figure in the updated README file.

With the read_screening.cpp script the user can use the inverse dielectric matrix contained in the other new .dat file to compute the exciton using the RPA screened potential.

The new version of xatu.cpp can compute the dielectric matrix in the BZ mesh, supporting both 2D and Q2D approaches.