TDM_DOWNDATE - renews a text collection by downdating the 
  correspoding term-document matrix
    A = TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS) returns the new 
    term - document matrix of the downdated collection. 
    UPDATE_STRUCT defines the update structure returned by TMG, 
    while REMOVED_DOCS defines the indices of the documents that 
    is to be be removed. 
    [A, DICTIONARY] = TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS) 
    returns also the dictionary for the updated collection, while 
    [A, DICTIONARY, GLOBAL_WEIGHTS, NORMALIZED_FACTORS]
    = TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS) returns the 
    vectors of  global weights for the dictionary and the 
    normalization factor for each document in case such a factor 
    is used. If normalization is not used TDM_DOWNDATE returns a 
    vector of all ones. [A, DICTIONARY, GLOBAL_WEIGHTS, 
    NORMALIZATION_FACTORS, WORDS_PER_DOC] = 
    TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS) returns statistics 
    for each document, i.e. the number of terms for each document. 
    [A, DICTIONARY, GLOBAL_WEIGHTS, NORMALIZATION_FACTORS, 
    WORDS_PER_DOC, TITLES, FILES] = TDM_DOWNDATE(UPDATE_STRUCT, 
    REMOVED_DOCS) returns in FILES the filenames containing the 
    collection's documents and a cell array (TITLES) that contains 
    a declaratory title for each document, as well as the document's 
    first line. 
    Finally [A, DICTIONARY, GLOBAL_WEIGHTS, NORMALIZATION_FACTORS, 
    WORDS_PER_DOC, TITLES, FILES, UPDATE_STRUCT] = 
    TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS) returns the update 
    structure that keeps the essential information for the 
    collection' s update (or downdate).
    TDM_DOWNDATE(UPDATE_STRUCT, REMOVED_DOCS, OPTIONS) defines 
    optional parameters: 
        - OPTIONS.dsp: Displays results (default 1) or not (0) 
          to the command window.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos