Implemented WCS subcube definition 

The subcube can now be defined in two ways:
1) import.subcubeMode='pix' and import.subcube = [X1, X2, Y1, Y2, Z1, Z2]. In this case X1 and X2 are the zero-based pixel positions that define the boundaries of the subcube along the X axis. Same thing for Y and Z axes.
2) import.subcubeMode='wcs' and import.subcube = [X, Y, Z, RX, RY, RZ]. In this case X is the centre and RX the radius of the subcube along the X axis. Both X and RX must be given in units of the X axis. No units conversion is performed. For example, if the Z axis is in Hz then Z and RZ must be given in Hz and not in, e.g., km/s.