Add comments to summation checks for ScenarioMIP

[laurinks]

Purpose of this PR

1. Add comments to summation checks for ScenarioMIP to indicate why certain checks currently fail.
2. Fix unit checks for BTC2 mapping.

Checklist:

• I did not use Excel to open csv files or checked that no side-effects occur (changed values, many new quotation marks, …)
• I added any renamed piam_variable to renamed_piam_variables.csv to guarantee backwards compatibility.
• For REMIND variables, I used |+| notation consistently. This can be achieved automatically with updatePlusUnit().

It is recommended to have a look at the tutorial before submission.

[flohump]

Thanks for adding the comments and for fixing the unit bugs!