add support for linear interpolation for generateIIASASubmission

[fbenke-pik]

Purpose of this PR

This adds support for linear interpolation when generation an IIASA submission.

• mappings can now have an optional column interpolation, which can have the entry linear
• the entry refers to the variable, not piam_variable, as interpolation is performed after applying the mapping
• linear interpolation is performed using quitte::interpolate_missing_periods for the years between minimum and maximum year (usually 2005 and 2100)

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.

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

[fbenke-pik]

@orichters Let us know if our chosen approach makes sense to you, or if there is a better way to do it.

[orichters]

Looks good, just unsure if putting it into getMapping is ideal. Because if you follow this route to edit a template in R, you now have an empty interpolation column:

mappingdata <- getMapping("AR6")
...
write.table(mappingdata, "inst/mappings/test_mapping.csv", na = "",  dec = ".", sep = ";", row.names = FALSE, quote = FALSE)