NF - add best_float function

Function to select floating point type with highest precision, avoiding
useless np.longdouble type on windows.

Author: matthew-brett

nibabel/casting.py

@@ -451,3 +451,21 @@ def floor_log2(x):
         ip += 1
         rem //= 2
     return ip
+
+
+def best_float():
+    """ Floating point type with best precision
+
+    This is nearly always np.longdouble, except on Windows, where np.longdouble
+    is Intel80 storage, but with float64 precision for calculations.  In that
+    case we return float64 on the basis it's the fastest and smallest at the
+    highest precision.
+
+    Returns
+    -------
+    best_type : numpy type
+        floating point type with highest precision
+    """
+    if type_info(np.longdouble)['nmant'] > type_info(np.float64)['nmant']:
+        return np.longdouble
+    return np.float64

nibabel/tests/test_casting.py

@@ -4,7 +4,7 @@
 import numpy as np
 
 from ..casting import (float_to_int, shared_range, CastingError, int_to_float,
-                       as_int, int_abs)
+                       as_int, int_abs, best_float)
 
 from numpy.testing import (assert_array_almost_equal, assert_array_equal)