TST: relax test for streamline length

Use almost equal test for alternative streamline length test.
RF: some PEP8 cleanup of test module

Long lines wrapped.
Some empty lines removed.
Specific import of functions to be tested.
Merge pull request #456 from matthew-brett/approx-streamline-tests

MRG: relax threshold for failing test + cleanup

Relax threshold for failing test as suggested by Omar.

Closes gh-455.

Clean up some PEP8 formatting.
BF: isinf is not defined in visual C. Using cimport instead.
BF: using cimport from libc.math instead of math.h in vector_fields and expectmax
BF: compare the return value of isinf against zero. Apparently it is returning a python Boolean instead of a simple int (I don't have this problem on my side though, if this doesn't work we may need to acquire the gil)
BF: using numpy's isinf with gil. Apparently isinf is not defined in libc.math
BF: imports math.h functions via dpy_math.h
BF: replaces isinf with npy_isinf
TEST: change circle-to-C test images' format from png to npy to remove dependency on matplotlib
Merge pull request #443 from omarocegueda/fix_isinf

MRG: Fix buildbots errors introduced with the registration module

Most failures are caused because "__isinf" is not defined in Visual C. Use imports from `dpy_math` instead.

Remove dependency on matplotlib by using `npy` files for data instead of `png` files.