Separate SIGSCR parser from the format

The code in SIGSCRFile.cs currently handles both the parsing and format for signal script files, but they should be split. The parser code will tokenise the file which will then be fed in to the format code to validate syntax and determine meaning. This is similar to the way all the Simis formats use STFReader to parse (tokenise) and separate code to handle the format.

Even though there will be only one format using the parser, maintenance will be easier and the code is likely to be simpler overall.