Do not create empty vertices during GFA load until needed #134

Job	Run time
🚧 Build SPAdes	1m 32s
Nextera L.monocytogenes	5m 23s
Multicell B.cereus TruSeq 100x	4m 27s
Full run M.ruber JGI lane 9	10m 46s
Contigs E.coli JGI lane 7	3m 50s
IonTorrent E.coli K12	29m 15s
Mate-pairs B.faecium	2m 13s
metaSPAdes JGI 150	15m 20s
metaviralSPAdes ERR525599	15m 59s
PacBio M.ruber	9m 7s
plasmidSPAdes C.callunae	7m 55s
Saves E.coli 100K	46s
Saves E.coli UCSD lane 1	8m 47s
Single-cell P.heparinus JGI lane 7	2m 59s
Single reads E.coli Deeptrench	8m 6s
	2h 6m 25s

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