Support for multiple analysis data sets in modules.

Main changes:
- Support for multiple data sets in the input data for most analysisdata
  modules.  For the clearest output, standard deviation values were
  moved from a separate column into the error value instead, which
  required updating a lot of unit test reference data.
- Support for computing multiple sets of angles using 'gmx gangle'.
- Extend the analysis data test utilities to handle multiple data sets
  when comparing against reference data.
- Add some unit tests for the new functionality.

Supporting changes:
- Add support for providing error values through AnalysisDataTestInput.
- AbstractArrayData now provides the means to use the full functionality
  of AnalysisDataValue for constructing the data.  AnalysisDataValue
  objects were already used internally for storage, so this amounts to
  exposing accessors for those objects instead of wrapping them in
  simple setters/getters for the value only.

Part of #1010.

Change-Id: I63abc05ab7e24362169e7d9ba0fa6ffc561be9d6