* positioning file(.pos.mat) format

This file preserves positioning result.
Format is below.
File accessor is stored $POSITIONING/file_accessor/posfile


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 Common fields
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header
    .file_type      : file type    [string] = ('POS-MAT')
    .version        : file version [double]
    .base_file_name : fitting result filename [string]

mri_key             : MD5 hash key made from analyze image file(*.img) [string]

spherical_info      spherical model information
    .Vcenter        : center of the sphere model   [1x3 double]
    .coord_type     : coordinate system of Vcenter [string] ('SPM_Right_m')
    .Vradius        : radius of the sphere model   [double]

trans_info

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 Other fields
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*** EEG case
coord_type             : coordinate system of pos [string] (='SPM_Right_m')
pos                    : channel position on MRI coordinate system [Nx3 double]
name                   : channel name list        [Nx1 cell]
trans_info
    .trans_mri         : affine transformation matrix [4x4 double] 
                         from Digitizer coordinate to MRI coordinate.
    .coord_type_before : coordinate system name before transform [string] (='digitizer_Right_m')
    .coord_type_after  : coordinate system name after transform  [string] (='SPM_Right_m')

*** MEG case
pos field is empty, name field is none.
only including transformation matrix.
(We suppose that sensor positions are stored in MEG file.)

coord_type             : coordinate system of pos [string] (='Unknown')
pos                    : empty (=[])
trans_info
    .trans_mri         : affine transformation matrix [4x4 double] 
                         from MEG sensor coordinate to MRI coordinate.
    .coord_type_before : coordinate system name before transform [string] (='Unknown')
    .coord_type_after  : coordinate system name after transform  [string] (='SPM_Right_m')
