TOF time shifting

[briantoby]

The time values for TOF datasets are traditionally reported as the value at the beginning of the bin. This means that the bin is best described with the Q/d-space value for the time half-way between the time labeling the bin and the next bin value. GSAS & GSAS-II have historically applied a shift to each time when histograms are read into the programs. Thus there are two different time definitions used within GSAS-II, the values as-read, with the time at the lower bin edge and the values used internally, with the time at the bin center.

This made life confusing when TOF values are used from GSAS-II. Since if a histogram was read and then exported the time values all changed. This has been addressed (see #251) in GSAS-II v5.6.0+ so that bin-edge values are now used for both import and export, so that bin-center values are pretty much isolated to internal use. GSASIIscriptable now provides access to both the internal (shifted to bin center) and with a new option (X-orig) to as-read (bin edge) times in the G2PwdrData.getdata() method. Scripts written before this change will continue to work as before.