STYLE: Improve the warning message style

Improve the warning message style: avoid whitespaces due to string
literal line breaks in code.

Allows to show:
```
UserWarning: b0_threshold (value: 0) is too low, increase your b0_threshold. It should be higher than the lowest b0 value (5.0).
  warn("b0_threshold (value: {0}) is too low, increase your "
```

instead of:
```
UserWarning: b0_threshold (value: 0) is too low, increase your              b0_threshold. It should be higher than the lowest b0 value              (5.0).
    warn("b0_threshold (value: {0}) is too low, increase your \
```

dipy/core/gradients.py

@@ -297,9 +297,9 @@ def gradient_table_from_bvals_bvecs(bvals, bvecs, b0_threshold=50, atol=1e-2,
 
     # checking for the correctness of bvals
     if b0_threshold < bvals.min():
-        warn("b0_threshold (value: {0}) is too low, increase your \
-             b0_threshold. It should be higher than the lowest b0 value \
-             ({1}).".format(b0_threshold, bvals.min()))
+        warn("b0_threshold (value: {0}) is too low, increase your "
+             "b0_threshold. It should be higher than the lowest b0 value "
+             "({1}).".format(b0_threshold, bvals.min()))
 
     bvecs = np.where(np.isnan(bvecs), 0, bvecs)
     bvecs_close_to_1 = abs(vector_norm(bvecs) - 1) <= atol