func MakeSharedStringRefTable(source *xlsxSST) *RefTable {
	reftable := NewSharedStringRefTable()
	reftable.isWrite = false
	for _, si := range source.SI {
		if len(si.R) > 0 {
			newString := ""
			for j := 0; j < len(si.R); j++ {
				newString = newString + si.R[j].T
			}
			reftable.AddString(newString)
		} else {
			reftable.AddString(si.T)
		}
	}
	return reftable
}
MakeSharedStringRefTable() takes an xlsxSST struct and converts
it's contents to an slice of strings used to refer to string values
by numeric index - this is the model used within XLSX worksheet (a
numeric reference is stored to a shared cell value).
package xlsx
type RefTable struct {
	indexedStrings []string
	knownStrings   map[string]int
	isWrite        bool
}
func NewSharedStringRefTable() *RefTable {
	rt := RefTable{}
	rt.knownStrings = make(map[string]int)
	return &rt
}