More fixing of the grammar and inclusion of single-quote quoted strings as well as double-quote ones.
updated the grammar to be a little more sensible

added a new class ExcelCell to represent a cell and provide helpers on relative cell addresses
tweaked some of the grammar to encapsulate term and function elements in tuples, rather than lists. Makes the parse tree easier to read and represents the data little better (I think)
started to implement the ability to walk the dependency tree