Improve 'g_ana help'.

Improve 'g_ana help'.

- 'g_ana help' now internally uses the new online help classes.
  Without any additional arguments, it still prints out the list of
  commands, and now also list of additional help topics.
- 'g_ana help <command>' prints help for the given command (the same as
  'g_ana <command> -h' for the trajectory analysis modules).
- 'g_ana help selections <subtopic>' can be used to access the general
  help for selections.

IssueID #666.

Change-Id: Id34044842b297591923bef242a93b4ae1ae9e14e