Convert gmx-sas to the C++ framework

Convert gmx-sas to the C++ framework

An example of converting a non-trivial analysis tool to the new
framework.  Most functionality is intact, but the command-line interface
is changed a bit.  Changes in behavior:
 - Writing a position restraint file is not implemented.  Would not be
   very difficult to add.
 - Solvation free energy estimates are written into a separate file
   instead of the main output file, and only if requested with an -odg
   option.
 - The calculation group is specified as a selection with -surface.
   Can be a dynamic selection.  The total area of this group is always
   written out.
 - There can be 0..* extra output groups, specified as selections with
   -output.  All can be dynamic, and must always be a subset of the
   calculation group.
 - There is no longer a separate concept of hydrophilic and hydrophobic
   surface area.  The user can trivially calculate such things by
   providing two different output groups.  The old hydrophobicity
   definition is easily expressed as "... and charge {-0.2 to 0.2}".
 - tpr file is no longer required for most of the output.
 - Output precision is different from the old tool.  The current
   precision (fixed to three decimal places) should be sufficient for
   most uses, but it could be considered to use %g for formatting like
   the old tool if really necessary.

Future TODOs:
 - Consider if the legends on the plots could be improved.
 - Add unit tests for the actual surface area calculation routine.

Part of #665.

Change-Id: Iee60c13b927b3b63b6e218164cd961971f2f3fce