Reduce the number of variables after nopo decomp standalone and adjust some labels

[mhamjediers]

Keep maybe only the variables that indicate strata, weight, and matching indicator; rename to indicate that they belong to nopo

Ensure that it still works also as a postestimation command from kmatch

[maximilian-sprengholz]

• Implemented with _nopo_matched, _nopo_mweight, and _nopo_strata as labeled returns
• kmatch generated vars are dropped by default, which can be overridden by specifying kmkeepgen as option
• _nopo_mweight and _nopo_strata are copies of the corresponding kmatch returns (you have doublettes if kmkeepgen is specified, but that approach keeps it clean and comprehensible)
• I added a check if the kmatch variables in e(generate and e(wgenerate) are still in the data when running nopo decomp - new calls after estimates restore kmatch do not work without kmkeepgen (in any case not the best idea, should be part of the help file)