API for pspm_pipeline_fc_scr is inconsistent

[gjheij]

• Creating dummy model file does not work if input is cell:

>> [pth, fn_m, ext] = fileparts(fn);
>> model_fn = fullfile(pth{1}, ['mdl_', fn_m{1}, '.mat']);
Brace indexing is not supported for variables of this type.

• isi{i_sn} would fail if input isi is scalar (it's not internally expanded)
• In general, would be good to verify that all inputs are cells (including missing)