fixed integration loop so that it doesn't overwrite initial data

In a local group discussion today, we found another correction to module 01_02. The Euler's method loop. Currently, the loop reads:
```python
z[0]=z0
# time-loop using Euler's method
for n in range(N):
    u = u + dt*numpy.array([u[1], g*(1-u[0]/zt)])
    z[n] = u[0]
```
But this overwrites z[0] with non-initial data.

Loop *should* read
```python
z[0]=z0
# time-loop using Euler's method
for n in range(1,N):
    u = u + dt*numpy.array([u[1], g*(1-u[0]/zt)])
    z[n] = u[0]
```
Thanks for all your hard work!