class_docstr: Base class for mystic functions
  
  The 'function' method must be overwritten, thus allowing
  calls to the class instance to mimic calls to the function object.
  
  For example, if function is overwritten with the Rosenbrock function:
      >>> rosen = Rosenbrock(ndim=3)
      >>> rosen([1,1,1])
      0.
     
docstr: takes a list of coefficients x, returns f(x)
docstr_tok: ["takes","a","list","of","coefficients","x",",","returns","f","(","x",")"]
code: def function(self, coeffs):
      """takes a list of coefficients x, returns f(x)"""
      raise NotImplementedError(
          "overwrite function for each derived class"
      )
  
file_path: mystic/mystic/models/abstract_model.py
imports: from numpy import sum
  from mystic.forward_model import CostFactory
label_desc: Use image node_id 3 for calling the AbstractFunction obj's underlying member method code with example usage: obj.function(coeffs) without return types