Merge your fork back into tonyqus/npoi

[LarsKemmann]

This repository is great in that there is now a .NET Standard version of NPOI available, but it's clearly not maintained so it's not seeing any of the benefits of upstream bugfixes or enhancements.

In the spirit of open source, can you please create a PR and merge your changes back into the main repository? It would help many of us!

Thanks!