BL-8703 better ePub metadata
BL-8703 better ePub metadata

* add isbn, copyright holder, copyright year to metadata
* use C#7.3 named tuples!
gmartin7/myBloomFork,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,StephenMcConnel/BloomDesktop