define fgb/lgb via t0 and offsets

a nice feature would be the option to define the fit- and data-range via t0 and offsets. This could be done the following way: In the GLOBAL block define, fgb_offset such that fgb = t0+fgb_offset, and lgb_range such that lgb = fgb + lgb_range. If this is defined, data(-range) would automatically be data fgb lgb, and fit(-range) would be fit fgb lgb.