Allow string formatting of reported differences for data points etc.

[dgm9704]

atm differences are reported in fixed format eg.
(a) Metric=mi125, Value=1910502575, Unit=iso4217:EUR, Decimals=-3, Context=A205 (APR=eba_AP:x42, BAS=eba_BA:x9, CPS=eba_CT:x22, EXC=eba_EC:x26, MCY=eba_MC:x195, PRP=eba_PL:x10, TRI=eba_TR:x4)
It would facilitate figuring these out in various systems if this was more flexible, eg. the format could be given as something like "$Dimension=$value, "