add parameter 'center' to the functions, and put indep variable x as first
update test file
update demo file
correct doc string
remove one space line
use better np function
add api_change file for physics_peak
better format
Merge pull request #128 from licode/function_correction

API : make the first argument to model functions the independent variable
remove extra space
new format
Merge pull request #129 from licode/function_correction

DOC : update api changes doc for model functions