Reads data from a fixed width textfile (i.e. the numbers are arranged in columns that are a given number of characters wide). Non-numerical data is converted in NaN. You can use this function for reading very large files in chunks, because you have to specify the line where to start reading (first line = 1), and the number of lines you wish to read.

Input:
- filename (string): filename to open (you have to include the extension)
- startline (int): line where to start reading
- number_of_lines (int): number of lines you want to read
- columns_width (vector): A vector containing for each column it's width. (The width of a column is the number of characters it is wide.)

Output: a matrix containing the read data.

01/05/2006, Adriaan Van Nuffel