Update BOM for Unicode text files

Read and change the Byte Order Mark (BOM) of a unicode file, as other Matlab functions which generate such text files do not add it.

When writing a text file with special characters (such as Hebrew, Russian or others), it is common to include a unique mark at the first bytes of the file.
However, Matlab functions such as writetable, that can be set to unicode, are not including this mark.
This function reads the file as binary, and extracts the marker. It can add it (if missing), change it, remove it, and write the result to the same file or to another one.