Flumpy doesn't handle non-owning-data numpy arrays

[ndevenish]

data = numpy.zeros(100)
>>> data[:] is data
False
>>> data[:].flags
  C_CONTIGUOUS : True
  F_CONTIGUOUS : True
  OWNDATA : False
  WRITEABLE : True
  ALIGNED : True
  WRITEBACKIFCOPY : False
  UPDATEIFCOPY : False

>>> flumpy.from_numpy(data)
<scitbx_array_family_flex_ext.double object at 0x7f50b463da50>
>>> flumpy.from_numpy(data[:])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'numpy.ndarray' object has no attribute 'obj'

This is because it assumes that if array.base exists, it is pointing to a memoryview object (e.g. something you previously converted). This isn't the case for standard slices of numpy arrays.