substring style function: 
- make sure that arguments are valid, else throw SyntaxException 
- make sure that string is long enough, else return null

Added unit tests 


git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3382 25d90789-57f7-4ee0-8453-03a3dfeeeb22
part style function: 

- make sure that arguments are valid, else throw SyntaxException 

Added unit tests 



git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3383 25d90789-57f7-4ee0-8453-03a3dfeeeb22