BF: fixed the dtype issue by forcing the type to np.float64
COSMIT: Cleaned test_field
COSMIT: a pass on test_graph
ENH: Avoid the systematic casting to np.float64 in dilation function
ENH: update docstring and remove one copy
Merge pull request #263 from bthirion/fix-type-graph

MRG: Fix type graph

This PR aims at solving issue #256 by casting to float 64 the arrays that need to.
I took this opportunity to clean the tests in nipy.algorithms.graph.