Make PatBBDTSeedClassifier compatible with recent ranges-v3

Merged Gerhard Raven requested to merge 2018-adapt-to-ranges-v3 into 2018-patches

Ranges-v3 has become more 'paranoid' wrt. InputRange vs. ForwardRange. As a result:

  • do not use front on an input range
  • use drop_exactly instead of tail for an InputRange
  • use cached instead of *begin
Edited by Gerhard Raven

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