Add support for cumulative histograms

Now the final, averaged histograms computed by the analysisdata
histogram modules can also be made cumulative.  This is needed to
preserve some functionality of 'gmx rdf' when moving to the C++
framework.  For now, only the unit test is using the function.

Change-Id: Iaf7175dd904f9a615b8bd887d677aa8dc1e10f0d