Optimized C code by moving a variable outside of a loop.

[thurinj]

It seems that the repeated access to sources(isrc,ig) causes a slow-down that can be easily avoided by caching the source as:

double source_val = sources (isrc, ig)

in which gives the following:

// Sum cross-correlations of all components being considered
for (ig=0; ig<NG; ig++) {
  double source_val = sources (isrc, ig);
  for (it=0; it<NPAD; it++) {
      cc(it) += greens_data(ista,ic,ig,it) * source_val;
  }
}

since sources (isrc, ig) only changes with the outer part of the loop.

From local tests on my personal M1 Mac, this change yields a speedup of about ~x1.18.

Given that this modifies a key component of the code, I'd like @rmodrak to look at it before we proceed.

🛠 Note: No tentative merge date -- will be merged only upon review.

[rmodrak]

Very helpful, looks good to me