support run mutiple queries in TPC-H benchmark

[zhangxffff]

for #81
Changes

• Support run mutiple queries in tpcbench.py for both Datafusion on Ray and Datafusion without Ray, so we can run all queries in tpch q15.
• Add --query argument for tpcbench.py to run custom query.
• modify docs/contributing.md.

[robtandy]

@zhangxffff thank you for submitting this!

I submitted one at the same time with a similar fix, combined with a few other small changes that came up during benchmarking. See #83

After that lands, though, can you add your support for --qnum though; that looks helpful.

[zhangxffff]

@zhangxffff thank you for submitting this!

I submitted one at the same time with a similar fix, combined with a few other small changes that came up during benchmarking. See #83

After that lands, though, can you add your support for --qnum though; that looks helpful.

I guess you mean --query here, I noticed that docs/contributing.md metions --query with tpc.py, but tpc.py was removed recently, so I add --query argument for tpcbench.py to run custom query with tpch tables. I will add --query support after your commits merged