0.18.1

This is a bug fix release:

• Fixes Issue #1180:
  get_pop_objs raised an AssertionError for the homogeneous case
  (income_percentiles=None with no income-specific inputs), because
  expand_pop_obj_J asserted before reaching its homogeneous branch.
  income_percentiles now defaults to a single income group (J=1) when
  no income-specific inputs are supplied, restoring the pre-0.18 call
  signature; it remains required whenever gradients or immigrant income
  shares are provided. Also removes a stray debug print from that
  branch.