moving imports of numpy (and matplotlib/mpl_toolkits) out of module scope and into functions/classes
reduces the import time of sympy by 50-75%.

See discussion at https://code.google.com/p/sympy/issues/detail?id=3427
add mpl_toolkits.mplot3d import to section of MatplotlibBackend for 3d plotting
Merge pull request #2140 from hacman/numpy_import

moving imports of numpy (and matplotlib/mpl_toolkits) out of module scope...