Convert gmx rdf to the C++ framework

Convert the RDF calculation tool to the new framework.  Main changed
beyond mechanical conversion:
 - Remove -com and -rdf options in favor of using selections directly.
   Both functionality, and a lot more, is now directly supported with
   proper selections.
 - Remove -hq and -fade options, as both are just post-processing that
   can be trivially done based on the output xvg file alone.
 - Normalization is reworked to work with dynamic selections.
 - Add an -rmax parameter that allows specifying an upper cutoff for the
   RDF.  Combined with the use of the neighborhood search routines, this
   potentially makes the tool a lot faster if the RDF is not required up
   to half the box length.

Add basic unit/regression tests for a simple water box.  The reference
values are not verified in any way, so they mainly catch regression
bugs.

Change-Id: Ieed19fc39a84e299291232190333652ced26abee