Gaussian fingerprintprimes calculation can be made faster

[Alireza Khorshidi]

In this line:

https://bitbucket.org/andrewpeterson/amp/src/6f8979e2ff7148541b3d976fe2c76649a7eb8c83/descriptor/gaussian.py?at=master&fileviewer=file-view-default#gaussian.py-876

and similar lines after, if der_position is identically equal to zero, it needs not be calculated. This can be decided based on the arguments input to der_position.

[Alireza Khorshidi]

This is now fixed.

[Alireza Khorshidi]

Andy: What kind of speed-up did this result in?

[Alireza Khorshidi]

This was solved in the commit be2de82, making the notions consistent with the paper.

"term3", "term5", ... here are new notions "dCosthetadRml", "dRijdRml", ... here. I put if statements such that if these values are identically equal to zero, then the calculations are skipped (adding a zero value to a number gives the same number).