Offset variable tapers are defined be picking times and offsets in a
seismogram display. It is quite likely that some of these picks, in particular
for near-source traces, are not within the time window of seismic sample data.
In such cases the four-point taper complaines and aborts. The offset variable
taper now ensures reasonable values to be passed to the four-point taper in
any case, by applying lower and upper bounds to the time values.