Merge branch 'fix_jupytext' into 'master'

execute.py

extra_markdown.tpl

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/3_drude_model.md

src/4_sommerfeld_model.md

src/5_atoms_and_lcao.md

src/6_bonds_and_spectra.md

src/7_tight_binding.md

src/8_many_atoms.md

src/9_crystal_structure.md

src/lecture_8.md