r116: removed code related to gapped ec

It is not working any more.
r117: compute k-mer coverage over ec read

This is used to count the number of "solid k-mer islands" (Heo et al, 2014),
which seems useful to identify bad reads.
r118: allow to optionally drop reads
r120: drop reads with too many failed k-mer trials

For human reads, the majority of false chimeric reads introduced in ec is
caused by this.