The negative prefix can now operate on ranges
Partially implemented the CELL function
Implemented the TRIM function
Implemented the MID() function
Don't let the add_function utility do anything with git. Causes problems with mac github client
Messed up with git somehow and didn't include implementation of CELL() in previous commit
Updated documentation
Give the simplification process access to the excel filename so that it can simplify out CELL() calls
Added the PV() function
Inlining now copes with references to missing worksheets