Add LoadCartesianGrid command

CARTESIAN GRID FILE FORMAT
------------------------------------------
    nx ny nz
    dx dy dz
    Ox Oy Oz
    zangle
    
    property1_name (continuous|categorical)
    <list of values of size nx*ny*nz>
    
    property2_name (continuous|categorical)
    <list of values of size nx*ny*nz>
    
    ...

Categorical properties aren't implemented yet, very busy at the moment...