r54: mark revision
also consider the original base
drop ambiguous k-mer in collection
r57: bugfix in r56
r58: change rules - drop k-mer with too many occ

don't drop ambiguous k-mers. They still help to correct errors.
r59: fixed a compiling error
r60: use a combination of max depth and ambiguity

if the depth is 5X of the average but unambiguous, we can still correct errors.
r61: don't drop ambiguous k-mer

This makes things worse. Also check max_ec_depth and q1_depth in
fmc_intv2tip(), not in the table.