FT_INSIDE_VOL locates dipole locations inside/outside the source
 compartment of a volume conductor model.

 Use as
   [inside] = ft_inside_vol(dippos, headmodel, ...)

 The input should be
   dippos      = Nx3 matrix with dipole positions
   headmodel   = structure with volume conductor model
 and the output is
   inside      = boolean vector indicating for each dipole wether it is inside the source compartment

 Additional optional input arguments should be given in key value pairs and can include
   inwardshift = number
   grad        = structure with gradiometer information, used for localspheres
   headshape   = structure with headshape, used for old CTF localspheres strategy