following 938298e, fix MCWeigther and examples

- add MCWeighter::defaultXsecDir()
- following 938298e, call buildSumwMap() after construction. For
  example, in Susy3LepCutflow this happens within `Init()`
- following 938298e, drop now-obsolete arguments from the default
  constructor
- fix test_mcWeighter.cxx, where now we can run the test without
  the input tree (but the key is not in the cache before calling
  `buildSumwMap`)
MCWeighter: drop obsolete and wrong comments