[!118] move `COMPARE_RECORD` into UGDIM namespace

Merge branch 'cleanup/move-COMPARE_RECORD-into-UGDIM-namespace' into 'master'

ref:staging/dune-uggrid This silences a compiler warning from GCC when
building programs with LTO where GCC sees two definitions of COMPARE_RECORD
(for 2d and for 3d):

    ../gm/refine.cc:3450:8: warning: type ‘struct compare_record’ violates the C++ One Definition Rule [-Wodr]
    ../gm/refine.cc:3450:8: note: a different type is defined in another translation unit
    ../gm/refine.cc:3453:12: note: the first difference of corresponding definitions is field ‘elem’
    ../gm/refine.cc:3453:12: note: a field of same name but different type is defined in another translation unit
    ../ug/gm.h:1644:24: note: type ‘union ELEMENT’ should match type ‘union ELEMENT’

See merge request [!118]

  [!118]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/118