SCUBA2: call JCMT::Tau directly to get filter tau from WVM reading

With the CSO tau (TAU225) FITS header often storing a value of 0, it's
not possible to use that value to obtain the filter tau. The simplest
solution is to call JCMT::Tau::get_tau directly to obtain the filter
tau from the WVM-derived 225-GHz optical depth.