Adding custom variables for tas uncertainty

[LisaBock]

Description

Adding custom varaibles for uncertainty of tas

tasConf5: range of uncertainty to lower bound (5% qunatile)
tasConf95: range of uncertainty to upper bound (95% quantile)

• Closes/Fixes: Adding uncertainty variables for tas #923

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Before you get started

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

Checklist

• PR has a descriptive title for the changelog
• Code follows the style guide
• Circle/CI tests pass
• Codacy code quality checks pass

[jvegreg]

Nice! I added a couple tests to check that they are handled correctly. We do not have equivalent tests for other custom vars, but it is the easiest way to test this

[LisaBock]

Nice! I added a couple tests to check that they are handled correctly. We do not have equivalent tests for other custom vars, but it is the easiest way to test this

@jvegasbsc Thanks a lot!