- Nested unordered_map was the cause of a significant memory/cell spike.
- added cell_reference hash, lookup by cell_reference to make memory usage proportional to cell count only
workbook::operator== was comparing the value of the raw pointer held by two std::unique_ptr's. By definition, this is always false in a well behaved program (if it's true, things go bang...). This then led to adding equality operators to nearly every other struct/class in xlnt to support workbook::operator==
workbook::load and the non-default ctors for loading data from a file are tested using the now functional equality operator
NOTE: a large number of copy ctors need updates/fixing. Many should be defaulted
