Support for default values for SelectionOption

SelectionOption now provides a defaultSelectionText() method to define a
default selection text for cases where the user does not provide the
option at all.  The free volume tool now uses this, since typical usage
is to run for all atoms in the system.
Was discussed on gmx-developers some time ago, so hopefully this is
useful in other contexts as well.
The default value does not appear in the output of -h, but that is
better fixed separately as part of a larger overhaul of the help output.

Change-Id: Ib4cba901a33e402a96ece473f70ac10365abcaac