marberts/sps: 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).