 TXT2MD - Creates an MD file from an ASCII text data file
    TXT2MD sfile dfile schema <keywords>
 Parameters:
    sfile  | source ASCII text file name and extension
    dfile  | destination MD file number
    schema | MD file schema name, e.g. IRAB, ISFC
 Keywords:
    MISS=  | value representing missing data in sfile
    ROW=   | row to write the data (def=first available)
 Remarks:
    TXT2MD is used to convert files containing lists of data to
    MD files.

    The source file (sfile) must be in ASCII text format, i.e.,
    created with an editor that stores ASCII text with a new line
    specifier at the end of each line.

    The format of sfile must be columnar with the first line
    containing the names of the schema keys.  Subsequent lines must
    contain data values corresponding to the schema keys.  For
    example, the first 3 lines could be:
      TIME  ID   LAT    LON    T       PSL      DIR  SPD
       12   MSN  43.13  89.33  280.42  1020.95  270  12
       12   MKE  42.95  87.90  278.04  1020.3   290  14
    Extra spaces between the columns are ignored.

    Each data line in sfile is placed into one record in the
    destination MD file (dfile).  Each data line must also include
    any data to be stored in the record's repeat groups (if repeat
    groups are defined in the schema).

    dfile will be created if it does not exist.  If dfile already
    exists, the data will be appended to the first available row or
    to that specified in ROW=.  When ROW= is used, any columns in
    the dfile row that match the column headers from sfile will be
    replaced.  If the column headers in sfile do not match any column
    headers in dfile, the data will be appended to new columns at the
    end of the row.

    The values for the row header keys (e.g., DAY, TIME) in dfile
    are set to the values found in the first data line of sfile.
    Values of the row header keys in subsequent data lines in sfile
    are ignored.

    The dfile must not exceed 6 column header keys or 3500 columns.
    Each record in dfile, including any repeat groups in that record,
    may not contain more than 64 keys.
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