code: def linear(target, m, b, prop):
      r
      x = target[prop]
      value = m*x + b
      return value
docs: r"""
      Calculates a property as a linear function of a given property
  
      Parameters
      ----------
      target : OpenPNM Object
          The object for which these values are being calculated.  This
          controls the length of the calculated array, and also provides
          access to other necessary thermofluid properties.
  
      m, b : floats
          Slope and intercept of the linear corelation
  
      prop : string
          The dictionary key containing the independent variable or phase
          property to be used in the correlation.