Statistics for gmx-distance.

Add support for calculating statistics more or less like g_bond has into
the new C++ gmx-distance tool.  The output format may not be as nice,
but one can at least get the same numbers out.

Extended the averaging analysisdata modules to handle multiple data sets
to support this, which required changing the data structure slightly
(this is the source of most modifications to existing data in XML files;
new tests/new output data of course adds additional entries).
Supporting changes in the plotting to handle missing values and errors,
and in test code to handle missing data points.

Part of #665.

Change-Id: If5f0b54d6d3eec0145625de9ab745e6ba9dc468a