Function correction

Done with the following corrections on physics peaks.

1. add center position, such as gauss_peak(area, std, dx) --> gauss_peak(x, area, center, std).

2. put independent variable x as the first argument to the function. I suggest we use this format.
A function to know is `np.zeros_like(x)` and `np.ones_like(x)` which give you back arrays matching the size (and I think dtype) of `x`.