Adding parquet functionality and filter injection for ark.

[1beb]

TODOs:

• [x] Identify if unark is required - partitioned files may present a problem (open_dataset perhaps, instead of read_parquet). Mentioned in #3 that there's an "archivists" preference.
• [X] Test case that includes missings to see if table classes match (integer/chr vs. logical na for example): made irrelevant by unify_schemas = T
• [x] Test cases for filter_statement
• [x] Need to unlink(dir, TRUE) if overwrite = TRUE and streamable_table$extension == "parquet".
• [x] SImplify sink name passage, can remove strsplit by removing .parquet initially from path.
• [x] Fix tests
• [x] Review code of conduct, see if there are signatures/agreements required.
• [x] Discuss approach with @cboettig (especially need to add conditionals in other functions, ark, keep_open)

Questions:

• [x] Is unark() functionality required?
• [x] How do we pass chunk_size to write_parquet for extremely large tables? Answer: don't. let lines param dictate output chunk size. Answer: We don't. Chunks must be written manually.
• [x] How do we filter a table for a subset of interest if we're not interested in all data? Answer: There is no method currently, consider adding one, see #41

Outline:

1. Added function: streamable_parquet which imports the arrow functions following the other external packages
2. Functions appear to be designed for text based format. Making a connection object the defacto. However, parquet requires a named sink. Needed to add conditionals to ark con object for this purpose.
3. parquet files would never be written without header information, made adjustment to keep_open to accomodate this.

[1beb]

@cboettig this is ready for another round of review. Note my question about unark. I'm not sure how meaningful (or even desirable) it would be.

[cboettig]

:eyes: nice, looking promising here! just ping me when you're ready for a review!

[1beb]

@cboettig

I accidentally included some of the filter injection on this one. Quick question, do you agree with the filter injection filter (as a concept), and if so, can I combine these two into a single PR (filter + parquet)?

[cboettig]

yup, I noticed the injection filter was here too. I agree that in principle it's something we should be handling, at least as an option, so I'm happy to have it wrapped in to the same PR.

[1beb]

@cboettig ok, we're ready for a review.

[cboettig]

:rocket: nice work, all looks good here.