Fix link only cartesian calc

[ThomasHepworth]

This is a follow up to PR #1193 which fixed a bug with link only jobs. It looks as if I missed the cartesian join code in that PR.

This pull request adds:

• A fix for the cartesian join code, now allowing single dataframes with a source_dataset column to be run.
• Adds a _verify_link_only_job to assess whether a link_only job has valid dataframe(s). This means we get a more informative error if only a single df is passed. Historically, you'd end up with a divide by 0 error.

For the divide by 0 error, you can run a link job with a single dataframe and no source_dataset column (or only a single value in the column).

[github-actions]

Test: test_2_rounds_1k_duckdb

Percentage change: -7.8%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
849	2022-07-12	18:40:05	1.89098	1.87463	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1599	2023-04-27	10:48:21	1.77524	1.72841	(detached head)	ace12fd	Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz	2.3945 GHz	ace12fd

Test: test_2_rounds_1k_sqlite

Percentage change: -8.4%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
851	2022-07-12	18:40:05	4.32179	4.25898	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1601	2023-04-27	10:48:21	3.94387	3.90201	(detached head)	ace12fd	Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz	2.3945 GHz	ace12fd

Click here for vega lite time series charts

[RossKen]

Runs as expected (including throwing helpful warnings when the inputs are incorrect) and the changes make sense.

Happy to approve, but if you can remove that stray if statement in misc.py that would be ace