Simulation/PIMixin: Allow specifying ensemble member to read
Although simulation mode only makes sense for a single ensemble member,
PI-XML files can contain multiple ensemble members. PIMixin in
simulation would always read the first ensemble member before this
commit. Instead of forcing the user to write a separate file for each
ensemble member, we now allow a new attribute pi_ensemble_member
that
specifies what ensemble member should be read from the PI-XML file.