Kmer: Add getFirstBase and getLastBase
DBGFM: Add simple edges

Add edges for the (k+1)-mer found in the original sequence.
DBGFM::vertex_iterator: Implement operator=
DBGFM: Performance improvement in addSimpleEdges

Cache the vertex_iterator.
DBGFM: Assemble contigs from tail to head

Follow the in-edges. Assembly is much faster because contigs are
assembled only once, not twice, and the k-mer are all oriented to
agree with the orientation of the contig.
DBGFM::assemble: Performance improvement

Update the suffix array interval at each step.
DBGFM: Use FMIndex::updateInterval