-- while unlikely to become an issue, ordering of ctors across source files is undefined and debugging issues related to it is not easy so just avoid that issue
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
