exercise 2 in 6-Balance is confusing and has the wrong implementation

```python
nAr = p/(kB*Tg) - y[:,0]- 0.5*y[:,2]- y[:,1]
```
should be
```python
nAr = p/(kB*Tg) - y[:,0]- 2*y[:,2]- y[:,1]
```
from argon conservation.

In the case of non-reacting gas, this would be:
```python
nAr = p/(kB*Tg) - y[:,0]- y[:,2]- y[:,1]
```
because of partial pressures, which makes the exercise a bit confusing.