Fix #634; expose point sets

[tianyikillua]

• Allow white lines between keyword lines
• Expose point sets, in line with Unification of point / cell sets among formats #619 (solves partially Abaqus parser #493, cell sets remain to be done)

<meshio mesh object>
  Number of points: 66
  Number of cells:
    quad: 50
  Point sets: _PickedSet2, _PickedSet7, _PickedSet8, _PickedSet14, _PickedSet15, _PickedSet16, _PickedSet17

In UUea.inp, we have

*Nset, nset=_PickedSet2, internal, generate
  1,  66,   1

this is read into

mesh.point_sets["_PickedSet2"] =
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16,
       17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,
       34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
       51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64],
      dtype=int32)

[gdmcbain]

This looks good. When I get time to take my next tilt at reading multiple Grids in XDMF #622, I'll follow this point_sets structure with the points in the corresponding Geometries.