Add support for w0 != -1 in config file, Class parameter file and Gadget code

[a-paranjape]

• Include w0 as parameter in config file.
• Include the following when writing the Class parameter file:

  if abs(w0 + 1) > small number:
  Omega_Lambda = 0 # force Omega_fld to be activated
  w0_fld = w0