class_docstr: Mostly a do nothing class. Lays out the methods expected of a sub-class.
  
  This is just a sketch of what we may want out of a general margins class.
  I (SS) need to look at details of other models.
code: def __init__(
      self, results, get_margeff, derivative, dist=None, margeff_args=()
  ):
      self._cache = {}
      self.results = results
      self.dist = dist
      self.get_margeff(margeff_args)
  
imports: from statsmodels.compat.python import lzip
  import numpy
  from scipy.stats import norm
  from statsmodels.tools.decorators import cache_readonly
label_desc: Use image node_id 1 to create a new Margins object  with example: obj = Margins(results, get_margeff, derivative, dist, margeff_args)