Implement Center/Radius algorithm

[neomonkeus]

See niftools/nifskope#69

• Implement centre/radius algorithm
• Set flags correctly on object

[HENDRIX-ZT2]

Consistency flags are set and retrieved correctly already. Center and radius of exported trishapes are set with suitable accuracy as well.
If anything, this is a pyffi issue, see this line in export_tri_shapes():
tridata.update_center_radius()

Looking at the source, pyffi does not check for mesh flags or version number:

pyffi/pyffi/formats/nif/__init__.py

Line 4417 in 7f4404d

def update_center_radius(self):

So this issue can be closed here. We might want to transfer it to pyffi to implement the nifskope-style version check there.