Fixes for the siconc variable of CMIP6 models

[remi-kazeroni]

Description

Closes #1104. Add missing scalar coordinate typesi for the variable siconc of CMIP6 models.

---

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🧪 The new functionality is relevant and scientifically sound
• 🛠 This pull request has a descriptive title and labels
• 🛠 Code is written according to the code quality guidelines
• 🧪 and 🛠 Documentation is available
• 🛠 Unit tests have been added
• 🛠 Changes are backward compatible
• 🛠 Any changed dependencies have been added or removed correctly
• 🛠 The list of authors is up to date
• 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

These fixes reuse what was done in #629.

[remi-kazeroni]

I moved the typesi fix into the shared fixes since it is needed for quite a few models. It's unclear to me if the value should be default or sea_ice.

def add_scalar_typesi_coord(cube, value='default'):

The few CESM2 examples with typesea or typeland are not so clear. That would help me to fix the failing test and add tests for other models.

[remi-kazeroni]

This is now ready for review. I have refactored things a bit to share the fix accross datasets.

[jvegreg]

Nice work!