add 2d test files

dune/alugrid/test/dgf/cube-testgrid-2-2.dgf

+DGF
+VERTEX
+0   0          
+0.5 0          
+1   0          
+0   0.5
+0.3 0.6
+1   0.5
+0   1
+0.5 1
+1   1.2
+#
+CUBE
+0 1 3 4
+1 2 4 5
+3 4 6 7
+5 4 8 7
+#
+

dune/alugrid/test/dgf/simplex-testgrid-2-2.dgf

+DGF
+
+VERTEX
+0.0000000000000000e+00 0.0000000000000000e+00
+1.0000000000000000e+00 0.0000000000000000e+00
+1.0000000000000000e+00 1.0000000000000000e+00
+0.0000000000000000e+00 1.0000000000000000e+00
+3.4999999999999998e-01 7.5000000000000000e-01
+2.9999999999999999e-01 8.3599999999999997e-01
+2.0000000000000001e-01 8.3599999999999997e-01
+1.4999999999999999e-01 7.5000000000000000e-01
+2.0000000000000001e-01 6.6300000000000003e-01
+2.9999999999999999e-01 6.6300000000000003e-01
+6.9999999999999996e-01 4.0000000000000002e-01
+6.5000000000000002e-01 4.8599999999999999e-01
+5.5000000000000004e-01 4.8599999999999999e-01
+5.0000000000000000e-01 4.0000000000000002e-01
+5.5000000000000004e-01 3.1300000000000000e-01
+6.5000000000000002e-01 3.1300000000000000e-01
+5.0000000000000000e-01 1.0000000000000000e+00
+0.0000000000000000e+00 5.0000000000000000e-01
+0.0000000000000000e+00 7.5000000000000000e-01
+2.5000000000000000e-01 4.8572758664047833e-01
+4.6250549019607845e-01 6.2747385620915030e-01
+2.5000000000000000e-01 1.0000000000000000e+00
+1.0000000000000000e+00 5.0000000000000000e-01
+5.0000000000000000e-01 0.0000000000000000e+00
+4.4519685039370083e-01 8.6288188976377955e-01
+0.0000000000000000e+00 6.2500000000000000e-01
+0.0000000000000000e+00 8.7500000000000000e-01
+3.7077300037721606e-01 2.6786379332023907e-01
+4.2054389993566155e-01 5.0373029073508058e-01
+6.0000000000000009e-01 6.1471492017248219e-01
+8.2922699962278390e-01 2.6786379332023924e-01
+8.2354545454545458e-01 5.2936363636363626e-01
+7.5000000000000000e-01 0.0000000000000000e+00
+9.6677461139896378e-02 6.6148704663212432e-01
+1.4993557735568527e-01 5.5319173918565745e-01
+0.0000000000000000e+00 2.5000000000000000e-01
+9.6690600522193221e-02 8.3852872062663186e-01
+1.2500000000000000e-01 1.0000000000000000e+00
+5.4337802736196594e-01 7.5176043560429240e-01
+1.0000000000000000e+00 7.5000000000000000e-01
+6.0000000000000009e-01 1.4451916932907344e-01
+1.1972352813036717e-01 4.0043948257617168e-01
+7.7174689682639375e-01 7.6589395416120554e-01
+#
+
+SIMPLEX
+27 0 23
+33 7 18
+11 29 12
+9 8 19
+19 28 9
+19 34 41
+6 5 21
+6 36 7
+24 16 21
+5 24 21
+25 33 18
+21 37 6
+28 12 20
+20 38 4
+36 37 26
+13 28 27
+14 27 40
+40 27 23
+15 40 30
+7 33 8
+24 4 38
+20 29 38
+10 30 31
+42 31 39
+5 4 24
+4 9 20
+14 13 27
+13 12 28
+29 20 12
+15 30 10
+34 19 8
+8 33 34
+3 26 37
+36 18 7
+41 34 17
+40 23 32
+20 9 28
+28 19 27
+10 31 11
+29 31 42
+30 1 22
+31 30 22
+38 29 42
+11 31 29
+32 1 30
+15 14 40
+17 34 25
+33 25 34
+41 17 35
+35 0 27
+19 41 27
+26 18 36
+22 39 31
+36 6 37
+24 38 16
+39 2 42
+38 42 16
+40 32 30
+41 35 27
+2 42 16
+#
+
+BOUNDARYSEGMENTS
+1   0 23
+1   11 12
+1   9 8
+1   6 5
+1   6 7
+1   16 21
+1   25 18
+1   21 37
+1   7 8
+1   5 4
+1   4 9
+1   14 13
+1   13 12
+1   15 10
+1   3 37
+1   3 26
+1   23 32
+1   10 11
+1   1 22
+1   32 1
+1   15 14
+1   17 25
+1   17 35
+1   35 0
+1   26 18
+1   22 39
+1   39 2
+1   2 16
+#
+
+#