Fix week_start on Snowflake

[clausherther]

week_start had a buggy and incomplete implementation on Snowflake. This PR intends to simplify and fix this.

Closes #53

[clausherther]

@jpmmcneill let me know if this makes sense to you. I think we had an incomplete macro there...

The other issue was that n_days_ago expects only integers, not expressions, so it was setting the offset to 0. :/

[davesgonechina]

Coincidentally I am also on Snowflake and noticed the week_start_date, week_end_date, and their prior counterparts were all null in the table I was generating with get_date_dimension, which calls week_start.

This PR works but only if you also follow Snowflake's recommendation to change from the legacy week_start==0 to, in my case, week_start==1.

[clausherther]

@davesgonechina my Snowflake setup is using week_start: 0 and this PR seems to work fine there?

[davesgonechina]

@clausherther and when you use get_date_dimension, are the week_start_date, week_end_date, and their prior_ counterparts populated? Because mine are not.

[clausherther]

@davesgonechina using this PR branch, here's what I see on my Snowflake instance after building all tables and running all tests:
(dates is built using get_date_dimension and dim_date comes from dates.

[davesgonechina]

@clausherther I take it back, you're right that it works with either parameter value, and solves the null column issue for get_date_dimension. Cheers!

[clausherther]

Going to go ahead and merge this and cut a new patch release. If there's something else, feel free to reopen the original issue (#53). Thanks!

[jpmmcneill]

Thanks @clausherther! Apols that I didn't contribute the PR, missed these notifications. I'll test it out today on the new patch and LYK how it's looking!

[jpmmcneill]

@clausherther verified working for me on 0.5.1. Thanks for the quick fix, i'd love to contribute to this package in future! I'll keep an eye on the issues that come in :)

[clausherther]

Thanks for checking @jpmmcneill! Sorry, didn't mean to jump in front on that one. I'm sure there'll be plenty of other bugs to squash in the future!