This fixes the segmentation fault when converting numpy and Eigen.
The reason is that NumpyEigenConverter.hpp includes numpy/arrayobject.h, and for
the numpy's C-API to work in multiple files we need to define NO_IMPORT_ARRAY
before including numpy/arrayobject.h in all the source files but the one that
defines the module initialization (exportgtsam.cpp in out case), as explained
here:
http://docs.scipy.org/doc/numpy/reference/c-api.array.html#importing-the-api
Note that PY_ARRAY_UNIQUE_SYMBOL, also needed to work multifile, is already
defined on NumpyEigenConverter.hpp.
Ellon Mendes