Improve space group support in .car files

- Support Hermann-Mauguin notation space groups with spaces and also
  the unspecified default space group.

- HM space group is enclosed within parentheses and can contain space
  between them as per the given format details in -
  http://hincklab.uthscsa.edu/html/soft_packs/msi_docs/insight980/formats980/ClassicFiles.html.

  The previous code didn't recognize such a spacegroup as it strictly
  looked for 8 fields after tokenizing them using default delimiters as
  space, tab, newline.

- It is legal to have .car files with PBC=ON and no space group defined
  in the fifth line and just have PBC and a,b,c,alpha,beta,gamma
  parameters. Such structure is considered to have lowest symmetry
  space-group as P1. This means there are no symmetry operations in play
  and all atoms are explicit within the unit cell. The .car files for
  H2O and polymers fall under this category since they are amorphous but
  are periodic.

- Added tests to ensure this is working.