class_docstr: Computes dual exponential decay [1].
  y = a1 + a2 Exp[-t / a4] + a3 Exp[-t/a5]
      
code: def __init__(self, name="decay", metric=lambda x: numpysum(x * x)):
      AbstractModel.__init__(self, name, metric)
      return
  
file_path: mystic/mystic/models/br8.py
imports: from .abstract_model import AbstractModel
  from numpy import array, asarray
  from numpy import sum
  from numpy import exp, sqrt
  from mystic.forward_model import CostFactory
label_desc: Use image node_id 1 to create a new BevingtonDecay object from inherited base classes: AbstractModel with example: obj = BevingtonDecay(name, metric)