Add command field for SPM

[thomasbtnfr]

The field command has been added for SPM as for FSL and AFNI.

For a specific matlabbatch like this one :

matlabbatch{13}.spm.stats.con.spmmat(1) = cfg_dep('Model estimation: SPM.mat File', substruct('.','val', '{}',{12}, '.','val', '{}',{1}, '.','val', '{}',{1}), substruct('.','spmmat'));
matlabbatch{13}.spm.stats.con.consess{1}.tcon.name = 'tone counting vs baseline';
matlabbatch{13}.spm.stats.con.consess{1}.tcon.weights = [1 0];
matlabbatch{13}.spm.stats.con.consess{1}.tcon.sessrep = 'none';
matlabbatch{13}.spm.stats.con.delete = 0;

the parser now stores the following content in the associated activity:

"command": "matlabbatch{13}.spm.stats.con.spmmat(1) = cfg_dep('Model estimation: SPM.mat File', substruct('.','val', '{}',{12}, '.','val', '{}',{1}, '.','val', '{}',{1}), substruct('.','spmmat'));\nmatlabbatch{13}.spm.stats.con.consess{1}.tcon.name = 'tone counting vs baseline';\nmatlabbatch{13}.spm.stats.con.consess{1}.tcon.weights = [1 0];\nmatlabbatch{13}.spm.stats.con.consess{1}.tcon.sessrep = 'none';\nmatlabbatch{13}.spm.stats.con.delete = 0;\n",

[cmaumet]

(I'll let you both @cyril-data @thomasbtnfr merge when possible -- i.e. when the PR this is depending on are merged and any conflicts resolved)