sps 0.6.0

• inclusion_prob() is now faster with a single stratum, improving the performance
  of sps() and expected_coverage().

• By default, sps_repweights() automatically picks the smallest
  tau to keep the replicate weights from being negative. Inspired by
  svrep::make_gen_boot_factors().

• Now requires R >= 4.1, which has been the case for a while (closing #2).