`grpstats` isn't MATLAB compatible

For Example:
Octave:
```matlab
[mC, g] = grpstats ([], [])
```
```matlab
error: 'gl_s' undefined near line 71, column 10
error: called from
    grp2idx at line 71 column 5
    grpstats at line 92 column 4
```

MATLAB:
```matlab
[mC, g] = grpstats ([], [])
```
```matlab
mC =

     []


g =

     []
```