code: def Cosine(x, a, omega, phi, y0):
      
      return a * np.cos(x * omega + phi) + y0
docs: Cosine function
  
      Inputs:
      -------
          ``x``: independent variable
          ``a``: amplitude
          ``omega``: circular frequency
          ``phi``: phase
          ``y0``: offset
  
      Formula:
      --------
          ``a*cos(x*omega + phi)+y0``