Minor fix for model name

Hi,

Taking the basename of the model filename as a basis for the model name is great. However, it broke the use of PepaModel without an associated file, which is useful. This was because os.path.basename was called on None in that case. Hence this fixes that. Now the name of the model is, in this order:

  1. The name supplied as the 'name' argument
  2. Based on the model file name if there is one
  3. A generic default if we have nothing else to go on.

cheers,
allan