Generic handling of online help topics.

Generic handling of online help topics.

- Added a few simple classes for handling a tree of help topics
  (src/gromacs/onlinehelp/).
- Rewrote the selection help to use the new classes. Required changing
  the help parser slightly to pass all requested help topics in a single
  call to _gmx_sel_handle_help_cmd(). Although this changes the usage of
  the help slightly, did not yet update the help texts.
- Added a way to specify the bison and flex binaries to use for
  regenerate_parser.sh (useful, e.g., if multiple versions are
  installed).

Related to #666.

Change-Id: I318fc0fc42e4af39734a5aa65a503582302ecdd8