Added warnings for 'slow' cluster size option

Changed snpm_cp.m and spm_append_96.m, so that when SnPM_ST file
approaches 1GB a (1-time) warning is issued.
Camille - I haven’t tested this!  Can you please?
Added warning for 'slow' cluster size if inference fails
Display msg if Locs_vox is not integer
Update version number to SnPM13.08