"We can do better" edition 2019

• all element types except sref and aref now have the datatype (dtype)
  property, which seems to be common practice (thanks to Eng Wen Ong
  for pointing it out).
• corrected the maximum length of structure names in gds_beginstruct.c
• fixes to build scripts. Octave > 5 no longer needs removal of .o files
• changed 'parfor' --> 'for' in 'adjmatrix.m' due to failure without Matlab Parallel Toolbox
• updated clipper library to version 6.4.2
• at long last fixed the bug in polygon area calculation
• fix bug in @gds_structure/add_ref.m (found by Daniel Contreras).
• fix comment in @gds_library/srename.m