* Changes to dSpacing to TOF in ConvertUnits

Changes to the Unit class and ConvertUnits algorithm to allow
diffractometer constants (DIFA, DIFC, TZERO) stored in the instrument map
to be used in conversions between d spacing and TOF and v.v.

Rather than add 3 more parameters to lots of methods provided by the Unit
class I have changed some of the methods to take a std:map containing the
optional parameters relevant to only some units. I have included efixed
and delta in this new map

New diffractometerConstants method added to SpectrumInfo and DetectorInfo
to allow retrieval for a particular spectrum including averaging across
detectors. Also manages situations where some of the three constants are
missing.

New algorithm created called ApplyDiffCal to read the diffractometer
constants from a diffraction calibration table workspace and write them
into the instrument parameter map

Add diffractometer constants to th...