I have downloaded the Apache Commons Math Package from their website, and extracted the .jar files.  Specifically, I took commons-math3-3.6.1.jar, and added it to my /Libraries/Java/Extensions directory ( on OSX ).  
I then added this .jar file as an External Jar in my Eclipse Project's Build Path.
What I would like to do is use the WeightedObservedPoints class. But when I write the following line:
import org.apache.commons.math3.fitting.WeightedObservedPoints;
I see the following error:
Access restriction: The type 'WeightedObservedPoints' is not API (restriction on required library '/Library/Java/Extensions/commons-math3-3.6.1.jar')
Does anybody know how I might solve this problem?  It doesn't make sense that this class would not be in the public API.  Am I missing a .jar file to add?  There were several in the original package, but they all seemed like documentation and other optional features like testing.
Thank you for any advice.
 
    