some a_func refactoring

[David-Baddeley]

Some small optimisations to weight matrix computation.

Most likely breaks things, as I was also playing with different weights. Changing lines 169-171 it should be easy enough to get back to original weights.

Optimisations:

• hard coded dimensionality (permits compiler loop unrolling/vectorisation)
• move some array accesses out of inner loop, try and reduce un-needed memory lookups
• PyMem_Calloc instead of calloc
• expf instead of exp (avoid cast, possibly faster as less precision needed)

[David-Baddeley]

overall gives a factor of 2-3 in compute_weight_matrix