Allow for creating models from Python strings

Following on from #67 I I think it would be a very good thing if MuMoT models could be created and referenced in Notebooks without using Notebook cell execution numbers. At present the example Notebooks contain cells like:

```
%%model
$
U -> A : g_A
U -> B : g_B
A -> U : a_A
B -> U : a_B
A + U -> A + A : r_A
B + U -> B + B : r_B
A + B -> A + U : s
A + B -> B + U : s
$
```
then
```python
model1 = mumot.parseModel(In[2])
```

This approach causes problems for the user if they want to re-run a given cell as then calls like `mumot.parseModel(In[X})` in subsequent cells won't work without manual editing of the cell execution number.

I suggest that what's needed is a Python function to generate a model from a multi-line string:

```python
model_1 = mumot.model_from_latex("""
U -> A : g_A
U -> B : g_B
A -> U : a_A
B -> U : a_B
A + U -> A + A : r_A
B + U -> B + B : r_B
A + B -> A + U : s
A + B -> B + U : s
""")
```

This is almost as readable as the but should allow Notebooks to be run more flexibly and robustly. You'd loose the LaTeX rendering beneath the `%%model` magic cells but that seems redundant given that `MuMoTmodel.show()` produces a more attractive rendering of that information.

Another advantage of allowing models to be generated by parsing a Python string is that it would make it easier to create models for unit testing outside Notebooks.