* First step extracting INDIRECT() and OFFSET() to their own classes
* Start building unit tests for OFFSET() and INDEX()
* Named ranges should be handled by the Calculation Engine, not by the implementation of the Excel INDIRECT() function
* When calling the calculation engine to get the range of cells to return, INDIRECT() and OFFSET() should use the instance of the calculation engine for the current workbook to benefit from cached results in that range
There's a couple of minor bugfixes in here; but it's basically just refactoring of the INDIRECT() and OFFSET() Excel functions into their own classes - still needs a lot of work on unit testing; and there's a lot more that could be improved in the code itself (including handling of the a1 flag for R1C1 format in INDIRECT()
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