Make quaternion.c conform with pure numpy C-API style

[moble]

From @moble on October 17, 2014 4:48

As demonstrated in, e.g., rational, it's considered good practice to define the member functions starting with

static NPY_INLINE 

to encourage the compiler to make various optimizations. This would only be applicable if the contents of that file are moved inside numpy_quaternion.c. Alternatively, I could use some macro for both, defining it to be inline if it's undefined at the top of quaternion.c, but defining it to be static NPY_INLINE just before including into numpy_quaternion.c.

Copied from original issue: moble/numpy_quaternion#2