* operator-- completes the naive bidirectional iterator requirements
* tests for various construction and assignment behaviours
* tests for iteration and dereferencing behaviours
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
-- custom_heights was the only test sheet where the integral value was being saved with a trailing ".0"
-- Updated tests to expect the new values
-- added the new property "defaultColWidth"
-- to encode: copy the string into unicodelookup.com. Take the hex representation and pad to 4 characters with '0's, prefixing each character with '\u'
-- sqref is a block of selected cells containing the active_cell (normally is just == active_cell)
-- active cell is the selected vell in this selection/view/pane
-- inactive selections use the top-left cell as the active_cell
-- NOTE: It doesn't seem likely that these should be 'A1' in all cases (set to 'A1' because current tests demanded it). Purpose of this parameter needs clarification
-- NOTE: [xlnt::selection] needs appropriate ctor's once the purpose of the parameters can be clarified
-- additional tests to cover both variants of the set function
-- additional tests to cover hyper links on cells with non-string data
-- modified hyper link set function to handle cells with non-string data
-- fixed test behaviour for a cell with no value (value is set to the string form of the link. Behaviour copied from Excel 2010)
-- Resolves two test failures caused by a crash when setting the title to the existing value removing the sheet from the 'sheet_title_rel_id_map_'
-- added empty title check
