Add sequential values test

[clrcrl]

This is a:

• bug fix PR with no breaking changes — please ensure the base branch is master
• new functionality — please ensure the base branch is the latest dev/ branch
• (possibly) a breaking change — please ensure the base branch is the latest dev/ branch

Description & motivation

This macro builds on work done in #264, to add a general test that a column has sequential values (i.e. an arithmetic series)

It's a little different to the exact functionality of that test, but I think this one is more generally useful

🚨 Note: when getting the tests passing, I noticed that the date_add macro has some strange behavior on BigQuery — we use datetime_add, and cast to a datetime rather than a timestamp (thus losing any timezone info). I thought it would be preferential to use timestamp_add here instead, which meant I ended up touching some other files. @jtcohen6 — I'd really like your eyes on that part of the code

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[jtcohen6]

I like it!

[jtcohen6]

LGTM!

[clrcrl]

huh, this works locally, and worked before I cleaned up this branch 🤔

[clrcrl]

:oof:

Database Error in model test_dateadd (models/cross_db_utils/test_dateadd.sql)
  TIMESTAMP_ADD does not support the MONTH date part at [36:9]

OK leaving this for now, it's chewing up too much time

[clrcrl]

Closing in favor of #327