Isthmus To/From SQL examples & enhanced APIs

[mbwhite]

Starting from two examples of using Isthmus to go from and to SQL a few new methods make the process a lot simpler and symetrical.

We found that in use we needed helper wrapper clases, but these could be pushed into the main code.

Note to reviewers

I'll leave as draft until I'm sure the build is good and tests are fully working.

I did create a separate examples project; however it does feel a bit unwieldy to have this and the spark one. Maybe some rework to make a general examples project with sub directories per example 'category'... Happy to make that change if you wish.

[nielspardon]

I did create a separate examples project; however it does feel a bit unwieldy to have this and the spark one. Maybe some rework to make a general examples project with sub directories per example 'category'... Happy to make that change if you wish.

I think it's fine having multiple example subprojects that each show how to use each of the 3 modules provided by substrait-java: Core, Isthmus, Spark. It would also show which dependency declarations / Gradle setup one needs to interact with each of the modules. If all of them were in a single examples project then it might be difficult to parse the Gradle setup and understand which dependencies / configuration is needed by which module.

[benbellick]

There were some parts I didn't quite get around to reading, but just dumping some comments I have for now. Thank you for doing this! I think it will be a great help :)

Most of my comments are about code comments / the readme. Feel free to ignore whatever if you think my suggestions are less clear. Thanks!

[benbellick]

Edited a bit for clarity

Suggested change

	In both cases, the Calcite library is used to do parsing and generation of the SQL String. Calcite has it's own relational object model, so there are classes within Isthmus to convert Substrait to and from Calcites object model.
	The [Calcite](https://calcite.apache.org/) library is used to do parsing and generation of the SQL String. Calcite has it's own relational object model, distinct from substrait's. There are classes within Isthmus to convert Substrait to and from Calcite's object model.

[mbwhite]

Thanks @benbellick - changes made. I've used the batch accepted feature - so hopefully I've not missed any comment - as I agreed with the updates.