switch to jupytext for markdown parsing

closes #74 (closed)

src/11_nearly_free_electron_model.md

-```{python initialize=true}
+```python tags=["initialize"]
 import numpy as np
 import plotly.offline as py
 import plotly.graph_objs as go
@@ -169,7 +169,7 @@ Let's consider a 1D crystal with a period $a$. Let $k_0$ be any wave number of a
         - Apply $\left<\phi_m\right|$ to the Schrödinger equation.
         - To evaluate $\left<\phi_m\right| \hat{H} \left| \phi_n\right>$, it may be helpful to separate the kinetic energy and potential energy of the Hamiltonian.
 
-5. Why is the dispersion relation only affected near $k=0$ and at the edge of the Brillouin zone (see also figures [above](#repeated-vs-reduced-vs-extended-brillouin-zone))? 
+5. Why is the dispersion relation only affected near $k=0$ and at the edge of the Brillouin zone (see also figures [above](#repeated-vs-reduced-vs-extended-brillouin-zone))?
 
     ??? hint
         To answer this question, only consider consider two free electron wavefunctions in the Hamiltonian and ignore all the others. Between what two of free electron wavefunctions does the coupling give significant contribution to the energy levels of the free electron wavefunctions?
@@ -190,4 +190,4 @@ Consider a 1D crystal with a periodic potential given by delta peaks: $$V(x) = -
 2. Make a sketch of the lower band.
 3. We now use the tight binding model, where we know that the dispersion relation can be described by $$E = \varepsilon_0 - 2 t \cos (ka).$$ Find an expression for $\varepsilon_0=\left<n\right| \hat{H} \left|n\right>$ and $-t=\left<n-1\right| \hat{H} \left| n \right>$, where $\left<x|n\right>$ represent the wavefunction of a single delta peak well at site $n$. You may make use of the results obtained in [exercise 2 of lecture 5](/5_atoms_and_lcao/#exercise-2-application-of-the-lcao-model) or [look up the wavefunction](https://en.wikipedia.org/wiki/Delta_potential).
 4. Compare the bands obtained in exercise 1 and 2: what are the minima and bandwidths (difference between maximum and minimum) of those bands?
-5. For what $a$ and $\lambda$ is the nearly free electron model more accurate? And for what $a$ and $\lambda$ is the tight binding model more accurate?
\ No newline at end of file
+5. For what $a$ and $\lambda$ is the nearly free electron model more accurate? And for what $a$ and $\lambda$ is the tight binding model more accurate?