Generated glue code should validate scalar

[szhorvat]

Currently, the generated glue code between R and C uses a very naive method to transfer scalar values to C. Since in R all values are vectors, the code simply takes the first element of that vector.

This should be improved, and it should be checked whether the data that was passed is indeed a scalar, i.e. whether it has length 1.

• The top priority is to ensure that the length is not zero, otherwise trying to take the first element may cause a crash, as in neighbors bug #153.
• It would also be good to check that the length is not greater than 1, but this would be a breaking change, as some other packages may rely on the current behaviour of just taking the first element.

[szhorvat]

Fixed by #1051

[github-actions]

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