%GETPREPROC Get datafile PREPROC field
%
% PREPROC = GETPREPROC(A)
%
% Retrieves the PREPROC field of the datafile A. This is a
% structure array with two fields:
% - preproc: a string with a command name
% - pars: a cell array with parameters
%
% [COMMAND,PARS] = GETPREPROC(A,N)
%
% In this case just the command name and the parameter cell array of
% the N-th preprocessing command are returned.
%
% or just the name of the N-th file.
% If N larger than the number of stored commands, COMMAND and PARS
% will be empty.