Fix 0.6s basin scaling in AG20
Response spectrum plotter shows that current USGS basin scaling algorithm misses 0.6s. Test if for T>0.5 s and then only 0.75s is explicitely scaled in tapering up to full basin effect at 1s. Could either just comment out 0.6s coefficients (not MPRS) or set test to be T>0.6s.
Edited by Powers, Peter M.