New dimfilter bug

**Description of the problem**

The following command works in GMT 6.3 but fails in GMT dev.
```
$ gmt dimfilter @static_earth_relief.nc -D4 -Fm600 -Gout.nc -Nl6 -R-55/-51/-24/-19      
dimfilter [ERROR]: Option -N: Correct syntax: -Nx<nsectors>[<modifier>], with x one of l|u|a|m|p, <nsectors> is number of sectors
```