Merge branch 'master' into 'crystal-structure-changes'

.mailmap

AUTHORS.md

execute.py

extra_markdown.tpl

mkdocs.yml

requirements.txt

src/11_nearly_free_electron_model.md

src/12_band_structures_in_higher_dimensions.md

src/13_semiconductors.md

src/1_einstein_model.md

src/2_debye_model.md

src/2_debye_model_solutions.md

src/3_drude_model.md

src/3_drude_model_solutions.md

src/4_sommerfeld_model.md

src/4_sommerfeld_model_solutions.md

src/5_atoms_and_lcao.md

src/5_atoms_and_lcao_solutions.md

src/6_bonds_and_spectra.md

src/6_bonds_and_spectra_solutions.md