Change how 1D graphs of Mantid data are plotted

[NickDraper]

From the Scientific steering committee.

The form of plotting we choose should depend on the underlying data in the workspace.

If the data is histogram data (nX=nY+1)
Default: we should plot the values at bin start up until bin end connecting together with horizontal lines. (ideally this option in MantidPlot should be renamed from Horizontal Lines to “Histogram”).
Optionally: If the users opts to connect the lines together as points then we should plot the bin value in the center of the bin (as we do now).

If the data is point data (nX=nY) we should plot at X value and connect with lines. (anyoptions for connecting with steps or Histogram should be greyed out)

[NickDraper]

This issue was originally trac ticket 459