Clean up batch postproc param recording

Loop through cell array of fields in param structure that should be
recorded in the csv output. If that field does not exist in the param
structure, enter a blank value. If it does, turn it into a string using
strjoin, and delimit it with ';' where necessary.
Add comments to batch mvpa post proc

Also change a few variable names, so that it is easier to inperpret the
code.