[MINC-development] Fix to netcdf external data types in libminc development branch

[Matthijs van Eede]

Greetings,

While compiling some minc tools using the development branch from libminc, I found that there was a netcdf external data type missing (NC_CHAR) in volume_io/Include/volume_io/volume.h, off-setting the values for the subsequent types (NC_SHORT, NC_INT... etc). I've created a patch and sent a pull request for it. 

Thanks!
Matthijs