Feature/decimal support

[GoEddie]

We are excited to review your PR.

So we can do the best job, please check:

• [x] There's a descriptive title that will make sense to other developers some time from now.
• [x] There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• [x] Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• [x] You have included any necessary tests in the same PR.

This implements https://github.com/dotnet/spark/issues/818

On the Apache Spark side decimal is implemented using a java.math.BigDecimal and the only way to construct that is using a int/long - there is no way to construct it with any value larger than max long unless you use the string constructor so I pass a string back and forth between .NET and the JVM, hope that is ok.

[Macromullet]

Can we get some traction on this one? It looks like this PR is ready and we have a lot of code that uses decimals that will be difficult to write tests around without this.

[Macromullet]

Any updates on this PR?

[AFFogarty]

Hey @GoEddie , are you still working on this?

[GoEddie]

Hi @AFFogarty,

I had given up really as no one seemed to be reviewing pr’s but am happy to get it up to date again.

ed