Showdata error handling

o Rather than exiting, showdata now raises errors.
  In an interactive session this will return to the
  prompt, rather than quitting entirely

o Checks for no inputs (Nvars == 0)

o Moved most documentation into function docstring,
  rather than module docstring

o To get time showdata collects "t_array" from current directory
  If this fails, t is now set to the time index using linspace