Speed up cNG calculation

[carlosggarcia]

We need to speed up the cNG calculation. At the moment it is unusable because it is too slow. @paulrogozenski measured the following in #114:

Time (seconds)
2h_22	700.271018505096
2h_13	76.4695343971252
3h	671.534012317658
4h	95.1586630344391
1h	84.9452083110809
cl_cov_cNG	5.12475728988647

We need to identify if is a tjpcov or ccl problem and fix it.