Update Readme and enhance code to deal with imperfect map data

[chrisl8]

The current revision of the code will not build with .NET Core 3.0, but 3.1 works, and that is the current LTS anyway.

[chrisl8]

1. I updated the Readme to point to the required .NET Core 3.1 to avoid people creating Issues when they cannot build it with the linked 3.0
2. My map was left with a lot of blank chunks due to the way the code bailed on entire chunks instead of for each block. I changed that.
3. I updated the code for every anomaly in my map that was causing it to fail. I don't know why my map behaves this way, but these updates should harden the code against the most difficult maps.

I've rendered a very large map with these changes and it works beautifully! Thank you so much for this amazing tool.