Support more complex fixed position selections

Support more complex fixed position selections

Make it possible to create selections like
  [0,0,0] plus [0,1,0]
to specify a fixed set of positions.  This can be useful at least for
'gmx gangle' to calculate angles from a fixed reference vector.
Such selections were already understood properly by the parser, but
caused various crashes elsewhere in the code.  Fixed those crashes by
consistently managing the memory for the involved t_block structures
(always allocate one value in the index array, even if the block is
empty), and by not assuming that a plus-like keyword always has a child
element in the evaluation tree.

Fixes #1619.

Change-Id: I513cddbe882f269ad867c726a54583ee48b41b4d