ENH: Define instance attributes in __init__ method

Define instance attributes in `__init__` method.

Fixes:
```
Instance attribute _datashape defined outside __init__
```

and similar warnings.

src/eddymotion/model/base.py

@@ -131,6 +131,9 @@ def __init__(self, gtab, S0=None, mask=None, b_max=None, **kwargs):
         module_name, class_name = model_str.rsplit(".", 1)
         self._model = getattr(import_module(module_name), class_name)(_rasb2dipy(gtab), **kwargs)
 
+        self._datashape = None
+        self._models = None
+
     def fit(self, data, n_jobs=None, **kwargs):
         """Fit the model chunk-by-chunk asynchronously"""
         n_jobs = n_jobs or 1
@@ -257,6 +260,7 @@ def __init__(self, **kwargs):
         self._th_high = kwargs.get("th_high", 10000)
         self._bias = kwargs.get("bias", True)
         self._stat = kwargs.get("stat", "median")
+        self._data = None
 
     def fit(self, data, **kwargs):
         """Calculate the average."""
@@ -329,6 +333,9 @@ def __init__(self, timepoints=None, xlim=None, n_ctrl=None, mask=None, order=3,
         # B-Spline knots
         self._t = np.arange(-3, float(self._n_ctrl) + 4, dtype="float32")
 
+        self._shape = None
+        self._coeff = None
+
     def fit(self, data, *args, **kwargs):
         """Fit the model."""
         from scipy.interpolate import BSpline