Filter mapping output based on R-squared

Make it so mapping functions output 0 if the R-squared of the fit is less than a desired threshold. This will minimise the number of voxels with non-sensical fits.

Add something like the snippet below to `mapping/fitting/relaxation.py` `fit_signal` function.

```python
if r2 <= 0:
    popt = np.zeros(model.n_params)
    error = np.zeros(model.n_params)
    r2 = -1E6
```
