improve+reduce fails+warnings in read+write functions; fix sorting order in quitteSort

[orichters]

1. If somehow an empty data.frame was written to a mif file and then read with read.quitte, the error message was incomprehensible:

Error in `if (last(period.columns) != length(header)) ...`:
! missing value where TRUE/FALSE needed
Run `rlang::last_trace()` to see where the error occurred.

Now, fail with

No column name found that could be understood as a 4-digit year.

2. let write.mif and write.IAMCxlsx try to convert to quitte instead of failing immediately, and warn in case of empty data

3. sort order in quitteSort according to function description, was somehow changed unnoticed in this commit

[0UmfHxcvx5J7JoaOhFSs5mncnisTJJ6q]

NB: unit is superfluous here, as variables can have only one unit by definition.

[orichters]

Only as long as you don't merge GDP (billion US$2010/yr) and GDP (billion US$2005/yr) data into one dataframe. Let us keep it.