Do not create empty vertices during GFA load until needed #135

Job	Run time
🚧 Build SPAdes	1m 57s
IonTorrent E.coli K12	29m 19s
Contigs E.coli JGI lane 7	3m 52s
Full run M.ruber JGI lane 9	12m 5s
Multicell B.cereus TruSeq 100x	4m 34s
Mate-pairs B.faecium	3m 27s
Nextera L.monocytogenes	1m 17s
metaSPAdes JGI 150	15m 24s
metaviralSPAdes ERR525599	15m 17s
PacBio M.ruber	4m 38s
plasmidSPAdes C.callunae	15m 59s
Saves E.coli 100K	44s
Saves E.coli UCSD lane 1	8m 53s
Single-cell P.heparinus JGI lane 7	2m 32s
Single reads E.coli Deeptrench	2m 40s
	2h 2m 38s

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