Use std::fill instead of assign

See http://stackoverflow.com/posts/8849789/revisions, with -03 on std::assign appears to be slower than std:fill
This patch results in unscientifically tested 1-2% speedup on a dataset with 306776 journey_pattern_point's
Merge pull request #445 from bliksemlabs/fill_vs_assign

Use std::fill instead of assign

Thank you for your hard work!