Merge branch 'master' of http://gitlab.onelab.info/gmsh/gmsh

restore mesh size test in edge collapse + fix collapse tests; this fixes several reported segfautls and brings back meshadapt performance closer to Gmsh 3 levels

better test for curve bounding boxes in case of multiple matches for periodicity constraints, when several curves have the same topology (cf. #1061)

Cgns write structured

See merge request !378

maximum simplification (here's nothing left ;-) before redoing interfaces, names and BC from scratch

just keep raw grid output for now; will need to decide how to save entity/physical names for structured grids

re-integrate refactored version of structured CGNS file output based on intial work financed by the Indian Institute of Technology Hyderabad - BRNS sponsored project in 2018 Under the guidance of Prof. Vinayak Eswaran <eswar@iith.ac.in>

this still contains the generation of boundary conditions, orientation checks, etc., that will probabably be removed