mark vertex_cells and halfedge_cells as const

7 jobs for master in 3 minutes and 37 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #14186
Linux
clang-c++11

00:02:24

passed #14188
Linux
clang-c++98

00:01:41

passed #14191
Linux
cppcheck

00:01:50

passed #14185
Linux
gcc-c++11

00:03:37

passed #14187
Linux
gcc-c++98

00:02:39

passed #14189
Apple
macos-c++11

00:02:30

passed #14190
Apple
macos-c++98

00:02:19