Sequential values v2

[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).

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